Light-emitting device, light-emitting apparatus, electronic device, and lighting device

ABSTRACT

A novel light-emitting device is provided. A light-emitting device with high emission efficiency is provided. A light-emitting device with along lifetime is provided. A light-emitting device with low driving voltage is provided. The light-emitting device includes an anode, a cathode, and an EL layer between the anode and the cathode. The EL layer includes a hole-injection layer, a light-emitting layer, and an electron-transport layer. The hole-injection layer is positioned between the anode and the light-emitting layer. The electron-transport layer is positioned between the light-emitting layer and the cathode. The hole-injection layer contains a first substance and a second substance. The first substance is an organic compound which has a hole-transport property and a HOMO level higher than or equal to −5.7 eV and lower than or equal to −5.4 eV. The second substance exhibits an electron-accepting property with respect to the first substance. The electron-transport layer contains a material whose resistance decreases with current flowing therethrough.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement, a light-emitting device, a display module, a lighting module, adisplay device, a light-emitting apparatus, an electronic device, and alighting device. Note that one embodiment of the present invention isnot limited to the above technical field. The technical field of oneembodiment of the invention disclosed in this specification and the likerelates to an object, a method, or a manufacturing method. Oneembodiment of the present invention relates to a process, a machine,manufacture, or a composition of matter. Specifically, examples of thetechnical field of one embodiment of the present invention disclosed inthis specification include a semiconductor device, a display device, aliquid crystal display device, a light-emitting apparatus, a lightingdevice, a power storage device, a memory device, an imaging device, adriving method thereof, and a manufacturing method thereof.

2. Description of the Related Art

Light-emitting devices (organic EL devices) including organic compoundsand utilizing electroluminescence (EL) have been put to more practicaluse. In the basic structure of such light-emitting devices, an organiccompound layer containing a light-emitting substance (an EL layer) isinterposed between a pair of electrodes. Carriers are injected byapplication of voltage to the device, and recombination energy of thecarriers is used, whereby light emission can be obtained from thelight-emitting substance.

Such light-emitting devices are of self-light-emitting type and thushave advantages over liquid crystal displays, such as high visibilityand no need for backlight when used as pixels of a display, and aresuitable as flat panel display devices. Displays including suchlight-emitting devices are also highly advantageous in that they can bethin and lightweight. Moreover, such light-emitting devices also have afeature that response speed is extremely fast.

Since light-emitting layers of such light-emitting devices can besuccessively formed two-dimensionally, planar light emission can beachieved. This feature is difficult to realize with point light sourcestypified by incandescent lamps and LEDs or linear light sources typifiedby fluorescent lamps. Thus, light-emitting devices also have greatpotential as planar light sources, which can be used for lightingdevices and the like.

Displays or lighting devices including light-emitting devices can besuitably used for a variety of electronic devices as described above,and research and development of light-emitting devices have progressedfor higher efficiency or longer lifetimes.

In a structure disclosed in Patent Document 1, a hole-transport materialwhose HOMO level is between the HOMO level of a first hole-transportlayer and the HOMO level of a host material is provided between alight-emitting layer and the first hole-transport layer in contact witha hole-injection layer.

Although the characteristics of light-emitting devices have beenimproved considerably, advanced requirements for various characteristicsincluding efficiency and durability are not yet satisfied.

REFERENCE Patent Document

-   [Patent Document 1] PCT International Publication No. WO2011/065136

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel light-emitting device. Another object of one embodiment of thepresent invention is to provide a light-emitting device with highemission efficiency. Another object of one embodiment of the presentinvention is to provide a light-emitting device with a long lifetime.Another object of one embodiment of the present invention is to providea light-emitting device with low driving voltage.

Another object of one embodiment of the present invention is to providea light-emitting apparatus, an electronic device, and a display deviceeach having high reliability. Another object of one embodiment of thepresent invention is to provide a light-emitting apparatus, anelectronic device, and a display device each with low power consumption.

It is only necessary that at least one of the above-described objects beachieved in the present invention.

One embodiment of the present invention is a light-emitting deviceincluding an anode, a cathode, and an EL layer between the anode and thecathode. The EL layer includes a hole-injection layer, a light-emittinglayer, and an electron-transport layer. The hole-injection layer ispositioned between the anode and the light-emitting layer. Theelectron-transport layer is positioned between the light-emitting layerand the cathode. The hole-injection layer contains a first substance anda second substance. The first substance is an organic compound which hasa hole-transport property and a HOMO level higher than or equal to −5.7eV and lower than or equal to −5.4 eV. The second substance exhibits anelectron-accepting property with respect to the first substance. Theelectron-transport layer contains a material whose resistance decreaseswith current flowing therethrough.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the material whose resistancedecreases with current flowing therethrough contains an organometalliccomplex of an alkali metal or an alkaline earth metal.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the material whose resistancedecreases with current flowing therethrough contains an organic compoundhaving an electron-transport property and an organometallic complex ofan alkali metal or an alkaline earth metal.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the organometallic complex of analkali metal or an alkaline earth metal forms a cluster.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the organometallic complex of analkali metal or an alkaline earth metal is a metal complex including aligand containing nitrogen and oxygen and an alkali metal or an alkalineearth metal.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the organometallic complex of analkali metal or an alkaline earth metal is a metal complex including amonovalent metal ion and a ligand having an 8-hydroxyquinolinatostructure.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the organometallic complex of analkali metal or an alkaline earth metal is a lithium complex including aligand having an 8-hydroxyquinolinato structure.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the electron-transport layer includesa first layer and a second layer. The first layer is positioned betweenthe light-emitting layer and the second layer. The second layer ispositioned between the first layer and the cathode. The concentration ofthe organometallic complex of an alkali metal or an alkaline earth metalin the first layer is different from that in the second layer.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the concentration of theorganometallic complex of an alkali metal or an alkaline earth metal inthe first layer is higher than that in the second layer.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the second substance is an organiccompound.

Another embodiment of the present invention is a light-emitting devicewith the above structure, in which the light-emitting layer contains ahost material and a light-emitting substance. The light-emittingsubstance emits blue fluorescence.

Another embodiment of the present invention is an electronic deviceincluding the light-emitting device with the above structure and asensor, an operation button, a speaker, or a microphone.

Another embodiment of the present invention is a light-emittingapparatus including the light-emitting device with the above structureand a transistor or a substrate.

Another embodiment of the present invention is a lighting deviceincluding the light-emitting device with the above structure and ahousing.

Note that the light-emitting apparatus in this specification includes,in its category, an image display device that uses a light-emittingdevice. The light-emitting apparatus may include a module in which alight-emitting device is provided with a connector such as ananisotropic conductive film or a tape carrier package (TCP), a module inwhich a printed wiring board is provided at the end of a TCP, and amodule in which an integrated circuit (IC) is directly mounted on alight-emitting device by a chip on glass (COG) method. Thelight-emitting apparatus may be included in a lighting device or thelike.

One embodiment of the present invention can provide a novellight-emitting device. Another embodiment of the present invention canprovide a light-emitting device with along lifetime. Another embodimentof the present invention can provide a light-emitting device with highemission efficiency. Another embodiment of the present invention canprovide a light-emitting device with low driving voltage.

Another embodiment of the present invention can provide a light-emittingapparatus, an electronic device, and a display device each having highreliability. Another embodiment of the present invention can provide alight-emitting apparatus, an electronic device, and a display deviceeach with low power consumption.

Note that the description of the effects does not disturb the existenceof other effects. One embodiment of the present invention does notnecessarily achieve all the effects listed above. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A1, 1A2, 1B, and 1C are schematic views of light-emittingdevices;

FIGS. 2A and 2B are diagrams for explaining a lifetime extension;

FIGS. 3A and 3B are diagrams for explaining a luminance increase;

FIGS. 4A and 4B are conceptual diagrams of an active matrixlight-emitting apparatus;

FIGS. 5A and 5B are conceptual diagrams of an active matrixlight-emitting apparatus;

FIG. 6 is a conceptual diagram of an active matrix light-emittingapparatus;

FIGS. 7A and 7B are conceptual diagrams of a passive matrixlight-emitting apparatus;

FIGS. 8A and 8B illustrate a lighting device;

FIGS. 9A, 9B1, 9B2, and 9C illustrate electronic devices;

FIGS. 10A to 10C illustrate electronic devices;

FIG. 11 illustrates a lighting device;

FIG. 12 illustrates a lighting device;

FIG. 13 illustrates in-vehicle display devices and lighting devices;

FIGS. 14A and 14B illustrate an electronic device;

FIGS. 15A to 15C illustrate an electronic device;

FIG. 16 shows an example of the Z plot of a light-emitting device of oneembodiment of the present invention;

FIG. 17 shows an example of the M plot of a light-emitting device of oneembodiment of the present invention;

FIG. 18 shows an equivalent circuit of the light-emitting device of oneembodiment of the present invention;

FIG. 19 shows the resistance values before and after the driving foreach of the resistances of the light-emitting device of one embodimentof the present invention.

FIG. 20 is a graph showing a change in luminance over driving time ofthe light-emitting device 1 and the comparative light-emitting device 1;

FIGS. 21A and 21B show the Z plots of the light-emitting device 1 andthe comparative light-emitting device 1;

FIG. 22 is a graph showing a change in voltage over driving time of thelight-emitting device 1 and the comparative light-emitting device 1;

FIG. 23 shows the M plot of the light-emitting device 1;

FIG. 24A shows the M plots of the light-emitting devices 2 to 5 and FIG.24B shows the resistance value for each resistance before and afterdriving;

FIG. 25A shows the M plots of the light-emitting devices 2, 6, and 7 andFIG. 25B shows the resistance value for each resistance before and afterdriving;

FIG. 26A shows the M plots of the light-emitting devices 2 and 8 andFIG. 26B shows the resistance value for each resistance before and afterdriving;

FIG. 27 shows the stabilized energy per molecule in a multimer of Liq;

FIG. 28 shows a measurement device.

FIG. 29 shows the calculated frequency characteristics of capacitance Cwhen a DC voltage is 7.0 V and a ratio of ZADN to Liq is 1:1;

FIG. 30 shows the frequency characteristics of −ΔB when a DC voltage is7.0 V and a ratio of ZADN to Liq is 1:1;

FIG. 31 shows the electric field strength dependence of electronmobility of organic compounds; and

FIGS. 32A1, 32A2, 32B1, and 32B2 each show the concentrationdistribution of an eighth substance in an electron-transport layer.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. Note that the present invention is notlimited to the following description, and it will be readily appreciatedby those skilled in the art that modes and details of the presentinvention can be modified in various ways without departing from thespirit and scope of the present invention. Therefore, the presentinvention should not be construed as being limited to the description inthe following embodiments.

Embodiment 1

FIGS. 1A1 and 1A2 each illustrate a light-emitting device of oneembodiment of the present invention. The light-emitting device of oneembodiment of the present invention includes an anode 101, a cathode102, and an EL layer 103. The EL layer includes a hole-injection layer111, a hole-transport layer 112, a light-emitting layer 113, and anelectron-transport layer 114. It is preferable that the hole-transportlayer 112 include a first hole-transport layer 112-1 and a secondhole-transport layer 112-2, and the electron-transport layer 114 includea first electron-transport layer 114-1 and a second electron-transportlayer 114-2 as illustrated in FIG. 1A2.

Although each of FIGS. 1A1 and 1A2 additionally illustrates anelectron-injection layer 115 in the EL layer 103, the structure of thelight-emitting device is not limited thereto. As long as theabove-described components are included, a layer having another functionmay be included.

The hole-injection layer 111 enables holes to be easily injected intothe EL layer 103, and is formed with a material with a highhole-injection property. The hole-injection layer 11 contains a firstsubstance and a second substance. The first substance is an organiccompound that has a hole-transport property and a relatively deep HOMOlevel higher than or equal to −5.7 eV and lower than or equal to −5.4eV. The second substance exhibits an electron-accepting property withrespect to the first substance. The first substance with a relativelydeep HOMO level inhibits induction of holes properly and facilitatesinjection of the induced holes into the hole-transport layer 112. Thehole-injection layer 111 having such a structure allows fabricating thelight-emitting device in which a carrier recombination region in thelight-emitting layer extends to the electron-transport layer at theinitial driving stage.

The second substance may be either an inorganic compound or an organiccompound; for example, an organic compound having anelectron-withdrawing group (in particular, a cyano group or a halogengroup such as a fluoro group) is preferably used. As the secondsubstance, a substance that exhibits an electron-accepting property withrespect to the first substance is selected from such substances asappropriate. Examples of such organic compounds include7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil,2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN), 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane(abbreviation: F₆-TCNNQ), and2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyren-2-ylidene)malononitrile.A compound in which electron-withdrawing groups are bonded to acondensed aromatic ring having a plurality of heteroatoms, such asHAT-N, is preferred because it is thermally stable. A [3]radialenederivative having an electron-withdrawing group (in particular, a cyanogroup or a halogen group such as a fluoro group) has a very highelectron-accepting property and thus is preferred. Specific examplesincludeα,α′,α″-1,2,3-cyclopropanetriylidenetris[4-cyano-2,3,5,6-tetrafluorobenzeneacetonitrile],α,α′,α″-1,2,3-cyclopropanetriylidenetris[2,6-dichloro-3,5-difluoro-4-(trifluoromethyl)benzeneacetonitrile],andα,α′,α″-1,2,3-cyclopropanetriylidenetris[2,3,4,5,6-pentafluorobenzeneacetonitrile].In the case where the second substance is an inorganic compound, atransition metal oxide can be used. In particular, an oxide of a metalbelonging to Group 4 to Group 8 in the periodic table is preferred. Asthe oxide of a metal belonging to Group 4 to Group 8 in the periodictable, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, rhenium oxide, or thelike is preferably used because their electron-accepting property ishigh. Among these oxides, molybdenum oxide is particularly preferablebecause of the stability in the air, low hygroscopic property, andeasiness of handling.

The first substance is preferably an organic compound having ahole-transport property and further preferably has any of a carbazoleskeleton, a dibenzofuran skeleton, a dibenzothiophene skeleton, and ananthracene skeleton. In particular, an aromatic amine having asubstituent that includes a dibenzofuran ring or a dibenzothiophene ringor an aromatic monoamine that includes a naphthalene ring, and anaromatic monoamine in which a 9-fluorenyl group is bonded to nitrogen ofthe amine through an arylene group are preferred. Note that the firstsubstance having an N,N-bis(4-biphenyl)amino group is preferable becausea light-emitting device with a long lifetime can be manufactured.Specific examples of the first substance includeN-(4-biphenyl)-6,N-diphenylbenzo[b]naphtho[1,2-d]furan-8-amine(abbreviation: BnfABP),N,N-bis(4-biphenyl)-6-phenylbenzo[b]naphtho[1,2-d]furan-8-amine(abbreviation:BBABnf),4,4′-bis(6-phenylbenzo[b]naphtho[1,2-d]furan-8-yl)-4″-phenyltriphenylamine(abbreviation: BnfBB1BP),N,N-bis(4-biphenyl)benzo[b]naphtho[1,2-d]furan-6-amine (abbreviation:BBABnf(6)), N,N-bis(4-biphenyl)benzo[b]naphtho[1,2-d]furan-8-amine(abbreviation: BBABnf(8)),N,N-bis(4-biphenyl)benzo[b]naphtho[2,3-d]furan-4-amine (abbreviation:BBABnf(1)(4)), N,N-bis[4-(dibenzofuran-4-yl)phenyl]-4-amino-p-terphenyl(abbreviation: DBfBB1TP),N-[4-(dibenzothiophen-4-yl)phenyl]-N-phenyl-4-biphenylamine(abbreviation: ThBA1BP), 4-(2-naphthyl)-4′,4″-diphenyltriphenylamine(abbreviation: BBAβNB),4-[4-(2-naphthyl)phenyl]-4′,4″-diphenyltriphenylamine (abbreviation:BBAβNBi), 4,4′-diphenyl-4″-(6;1′-binaphthyl-2-yl)triphenylamine(abbreviation: BBAαNβNB),4,4′-diphenyl-4″-(7;1′-binaphthyl-2-yl)triphenylamine (abbreviation:BBAαNβNB-03), 4,4′-diphenyl-4″-(7-phenyl)naphthyl-2-yltriphenylamine(abbreviation: BBAβANB-03),4,4′-diphenyl-4″-(6;2′-binaphthyl-2-yl)triphenylamine (abbreviation:BBA(βN2)B),4,4′-diphenyl-4″-(7;2′-binaphthyl-2-yl)triphenylamine(abbreviation:BBA(βN2)B-03), 4,4′-diphenyl-4″-(4;2′-binaphthyl-1-yl)triphenylamine(abbreviation: BBAβNαNB),4,4′-diphenyl-4″-(5;2′-binaphthyl-1-yl)triphenylamine(abbreviation:BBAβNαNB-02),4-(4-biphenylyl)-4′-(2-naphthyl)-4″-phenyltriphenylamine(abbreviation: TPBiAβNB),4-(3-biphenylyl)-4′-[4-(2-naphthyl)phenyl)-4″-phenyltriphenylamine(abbreviation: mTPBiAβNBi),4-(4-biphenylyl)-4′-[4-(2-naphthyl)phenyl]-4″-phenyltriphenylamine(abbreviation: TPBiAβNBi), 4-phenyl-4′-(1-naphthyl)triphenylamine(abbreviation: αNBA1BP), 4,4′-bis(I-naphthyl)triphenylamine(abbreviation: αNBB1BP),4,4′-diphenyl-4″-[4′-(carbazol-9-yl)biphenyl-4-yl]triphenylamine(abbreviation: YGTBilBP),4′-[4-(3-phenyl-9H-carbazol-9-yl)phenyl]tris(1,1′-biphenyl-4-yl)amine(abbreviation: YGTBilBP-02),4-diphenyl-4′-(2-naphthyl)-4″-[9-(4-biphenylyl)carbazol]triphenylamine(abbreviation: YGTBiβNB),N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-N-[4-(1-naphthyl)phenyl]-9,9′-spirobi[9H-fluoren]-2-amine(abbreviation: PCBNBSF),N,N-bis(4-biphenylyl)-9,9′-spirobi[9H-fluoren]-2-amine (abbreviation;BBASF), N,N-bis(1,1′-biphenyl-4-yl)-9,9′-spirobi[9H-fluoren]-4-amine(abbreviation: BBASF(4)),N-(1,1′-biphenyl-2-yl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9′-spirobi(9H-fluoren)-4-amine(abbreviation: oFBiSF),N-(4-biphenyl)-N-(dibenzofuran-4-yl)-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBiF),N-[4-(1-naphthyl)phenyl]-N-[3-(6-phenyldibenzofuran-4-yl)phenyl]-1-naphthylamine(abbreviation: mPDB(BNBN),4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP),4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation;mBPAFLP), 4-phenyl-4′-[4-(9-phenylfluoren-9-yl)phenyl]triphenylamine(abbreviation: BPAFLBi),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBAIBP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBilBP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation PCBANB),4,4′-di(I-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF), andN-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluoren-2-amine(abbreviation: PCBBiF).

The weight ratio of the first substance to the second substance in thehole-injection layer 111 is preferably 1:0.01 to 1:0.15, furtherpreferably 1:0.01 to 1:0.1.

The hole-transport layer 112 preferably includes the firsthole-transport layer 112-1 and the second hole-transport layer 112-2,though it may be a single layer. The first hole-transport layer 112-1 iscloser to the anode 101 side than the second hole-transport layer 112-2is. Note that the second hole-transport layer 112-2 also functions as anelectron-blocking layer in some cases.

The first hole-transport layer 112-1 and the second hole-transport layer112-2 each contain an organic compound having a hole-transport property.As the organic compound having a hole-transport property, the organiccompound that can be used as the first substance can be similarly used.Note that the first hole-transport layer 112-1 and the secondhole-transport layer 112-2 may include the same hole-transport organiccompound or different hole-transport organic compounds. Thehole-transport organic compound in the first hole-transport layer 112-1and the first substance in the hole-injection layer 111 may be the sameorganic compound or different organic compounds.

It is preferable that materials for the first substance in thehole-injection layer 111 and the hole-transport organic compound in thefirst hole-transport layer 112-1 be selected so that the HOMO level ofthe hoe-transport organic compound is deeper than that of the firstsubstance and a difference between their HOMO levels is less than orequal to 0.2 eV.

In addition, the HOMO level of the hole-transport organic compound inthe second hole-transport layer 112-2 is preferably deeper than that ofthe hole-transport organic compound in the first hole-transport layer112-1, and their materials are preferably selected so that a differencebetween the HOMO levels is less than or equal to 0.2 eV. Owing to such arelation between the HOMO levels of the hole-transport organic compoundsincluded in the hole-injection layer 111 to the second hole-transportlayer 112-2, holes are injected into each layer smoothly, which preventsan increase in driving voltage and deficiency of holes in thelight-emitting layer.

The first substance included in the hole-injection layer 111 and thehole-transport organic compounds included in the hole-transport layer112 (the first hole-transport layer 112-1 and the second hole-transportlayer 112-2) each preferably have a hole-transport skeleton. A carbazoleskeleton, a dibenzofuran skeleton, a dibenzothiophene skeleton, and ananthracene skeleton, with which the HOMO levels of the organic compoundsdo not become too shallow, are preferably used as the hole-transportskeleton. Materials contained in adjacent layers preferably have thesame hole-transport skeleton, in which case holes can be injectedsmoothly. In particular, a dibenzofuran skeleton is preferably used asthe hole-transport skeleton.

Furthermore, adjacent layers preferably contain the same organiccompound so that holes can be injected more smoothly. In particular, thefirst substance included in the hole-injection layer 111 and thehole-transport organic compound included in the first hole-transportlayer 112-1 are preferably the same material.

The light-emitting layer 113 contains an emission substance and a hostmaterial. Note that the light-emitting layer 113 may additionallycontain another material, and may be a stack of two layers withdifferent compositions.

The emission substance may be fluorescent substances, phosphorescentsubstances, substances exhibiting thermally activated delayedfluorescence (TADF), or other emission substances. Note that oneembodiment of the present invention is more suitable for the case wherethe light-emitting layer 113 emits fluorescence, specifically, bluefluorescence.

Examples of the material that can be used as a fluorescent substance inthe light-emitting layer 113 are as follows. Other fluorescentsubstances can also be used.

Examples include 5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine(abbreviation: PAP2BPy),5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn),N,N-bis[4-(9H-carbazol-9-yl)phenyl]-N,N-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA) perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N″-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,′″,N′″-octaphenyidibenzo[g,p]chrysene-2,7,10,15-tetramine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N″-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), counarin 545T, N,N″-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N″-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N∝,N″-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM),N,N-diphenyl-N,N-(1,6-pyrene-diyl)bis[(6-phenylbenzo[b]naphtho[1,2-d]furan)-8-amine](abbreviation: 1,6BfAPrn-03),3,10-bis[N-(9-phenyl-9H-carbazol-2-yl)-N-phenylamino]naphtho[2,3-b;6,7-b′]bisbenzofuran(abbreviation: 3,10PCA2Nbl(IV)-02), and3,10-bis[N-(dibenzofuran-3-yl)-N-phenylamino]naphtho[2,3-b;6,7-b′]bisbenzofuran(abbreviation: 3,10FrA2Nb(IV)-02). Condensed aromatic diamine compoundstypified by pyrenediamine compounds such as 1,6FLPAPrn, 1,6mMemFLPAPrn,and 1,6BnfAPrn-03 are particularly preferable because of their highhole-trapping properties, high emission efficiency, and highreliability.

Examples of the material that can be used when a phosphorescentsubstance is used as the emission substance in the light-emitting layer113 are as follows.

Examples include an organometallic iridium complex having a 4H-triazoleskeleton, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-dmp)₃]),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: [Ir(Mptz)₃]), andtris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: [Ir(iPrpt-3b)₃]); an organometallic iridium complexhaving a 1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: [Ir(Mptz1-mp)₃]) andtris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: [Ir(Prptz1-Me)₃]); an organometallic iridium complexhaving an imidazole skeleton, such asfac-tris[N-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(Ill)(abbreviation: [Ir(iPrpmi)₃]) andtris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: [Ir(dmpimpt-Me)₃]); and an organometallic iridium complexin which a phenylpyridine derivative having an electron-withdrawinggroup is a ligand, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C²]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis{2-(4′,6′-difluorophenyl)pyridinato-N,C²}iridium(III) picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C²}iridium(III)picolinate (abbreviation: [Ir(CF₃ppy)₂(pic)]), andbis[2-(4′,6-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIr(acac)). These compounds emit bluephosphorescence having an emission peak at 440 nm to 520 nm.

Other examples include an organometallic iridium complex having apyrimidine skeleton, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:[Ir(mppm)₃]), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₃]),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(mppm)₂(acac)]),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]),(acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: [Ir(nbppm)₂(acac)]),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: [Ir(mpmppm)₂(acac)]), and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]); an organometallic iridium complexhaving a pyrazine skeleton, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(acac)]) and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: [U(mppr-iPr)₂(acac)]); an organometallic iridium complexhaving a pyridine skeleton, such astris(2-phenylpyridinato-N,C^(2′))iridium(l) (abbreviation: [Ir(ppy)₃]),bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: [Ir(ppy)₂(acac)]), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: [Ir(bzq)₂(acac)]),tris(benzo[h]quinolinato)iridium(III) (abbreviation: [Ir(bzq)₃]),tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation:[Ir(pq)₃]), andbis(2-phenylquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation:[Ir(pq)₂(acac)]); and a rare earth metal complex such astris(acetylacetonato)(monophenanthroline)terbium(III)(abbreviation:[Tb(acac)₃(Phen)]). These compounds mainly emit green phosphorescencehaving an emission peak at 500 nm to 600 nm. Note that an organometalliciridium complex having a pyrimidine skeleton has distinctively highreliability and emission efficiency and thus is especially preferable.

Other examples include an organometallic iridium complex having apyrimidine skeleton, such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: [Ir(5mdppm)₂(dibm)]),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanate)iridium(III)(abbreviation: [Ir(5mdppm)₂(dpm)]), andbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanate)iridium(III)(abbreviation:[Ir(d1npm)₂(dpm)]); an organometallic iridium complex having a pyrazineskeleton, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: [Ir(tppr)₂(acac)]),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanate)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]), and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: [Ir(Fdpq)₂(acac)]); an organometallic iridium complexhaving a pyridine skeleton, such astris(1-phenylisoquinolinato-N,C^(2″))iridium(III) (abbreviation:[Ir(piq)₃]) and bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation:[Ir(pig)₂(acac)]); a platinum complex suchas 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and a rare earth metal complex such astris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: [Eu(DBM)₃(Phen)]) andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: [Eu(TTA)₃(Phen)]). These compounds emit redphosphorescence having an emission peak at 600 nm to 700 nm.Furthermore, an organometallic iridium complex having a pyrazineskeleton can emit red light with favorable chromaticity.

Besides the above phosphorescent compounds, known phosphorescentsubstances may be selected and used.

Examples of the TADF material include a fullerene, a derivative thereof,an acridine, a derivative thereof, and an eosin derivative. Otherexamples include a metal-containing porphyrin such as a porphyrincontaining magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum(Pt), indium (In), or palladium (Pd). Examples of the metal-containingporphyrin include a protoporphyrin-tin fluoride complex (SnF₂(ProtoIX)), a mesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrintetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (SnF₂(OEP)), anetioporphyrin-tin fluoride complex (SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (PtCl₂OEP), which arerepresented by the following structural formulae.

Alternatively, a heterocyclic compound having one or both of aπ-electron rich heteroaromatic ring and a π-electron deficientheteroaromatic ring that is represented by the following structuralformulae, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ),9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole(abbreviation: PCCzTzn),9-[4-[4,6-diphenyl-1,3,5-triazine-2-yl]phenyl]-9′-phenyl-9H,9′H-3,3′-bicarbazole(abbreviation: PCCxPTzn),2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine(abbreviation: PXZ-TRZ),3-[4-(5-phenyl-5,10-dihydrophenazin-10-yl)phenyl]-4,5-diphenyl-1,2,4-triazole(abbreviation: PPZ-3TPT),3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation:ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone(abbreviation: DMAC-DPS), or10-phenyl-10H,10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation:ACRSA) can be used. Such a heterocyclic compound is preferable becauseof having excellent electron-transport and hole-transport propertiesowing to a π-electron rich heteroaromatic ring and a π-electrondeficient heteroaromatic ring. Among skeletons having the π-electrondeficient heteroaromatic ring, a pyridine skeleton, a diazine skeleton(a pyrimidine skeleton, a pyrazine skeleton, or a pyridazine skeleton),and a triazine skeleton are preferable because of their high stabilityand reliability. In particular, a bezofuropyrimidine skeleton, abenothienopyrimidine skeleton, a benzofuropyrazine skeleton, and abenzothienopyazine skeleton are preferable because of their highacceptor properties and reliability. Among skeletons having theπ-electron rich heteroaromatic ring, an acridine skeleton, a phenoxazineskeleton, a phenothiazine skeleton, a furan skeleton, a thiopheneskeleton, and a pyrrote skeleton have high stability and reliability;therefore, at least one of these skeletons is preferably included. As afuran skeleton, a dibenzofuran skeleton is preferable. As a thiopheneskeleton, a dibenzothiophene skeleton is preferable. As a pyrroleskeleton, an indole skeleton, a carbazole skeleton, an indolocarbazoleskeleton, a bicarbazole skeleton, and a3-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole skeleton are particularlypreferable. Note that a substance in which the π-electron richheteroaromatic ring is directly bonded to the it-electron deficientheteroaromatic ring is particularly preferable because theelectron-donating property of the π-electron rich heteroaromatic ringand the electron-accepting property of the π-electron deficientheteroaromatic ring are both improved, the energy difference between theS1 level and the T1 level becomes small, and thus thermally activateddelayed fluorescence can be obtained with high efficiency. Note that anaromatic ring to which an electron-withdrawing group such as a cyanogroup is bonded may be used instead of the π-electron deficientheteroaromatic ring. As a π-electron rich skeleton, an aromatic amineskeleton, a phenazine skeleton, or the like can be used. As a π-electrondeficient skeleton, a xanthene skeleton, a thioxanthene dioxideskeleton, an oxadiazole skeleton, a triazole skeleton, an imidazoleskeleton, an anthraquinone skeleton, a boron-containing skeleton such asphenylborane or boranthrene, an aromatic ring or a heteroaromatic ringhaving a nitrile group or a cyano group, such as benzonitrile orcyanobenzene, a carbonyl skeleton such as benzophenone, a phosphineoxide skeleton, a sulfone skeleton, or the like can be used. Asdescribed above, a π-electron deficient skeleton and a π-electron richskeleton can be used instead of at least one of the π-electron deficientheteroaromatic ring and the π-electron rich heteroaromatic ring.

Note that a TADF material is a material having a small differencebetween the S1 level and the T1 level and a function of convertingtriplet excitation energy into singlet excitation energy by reverseintersystem crossing. Thus, a TADF material can upconvert tripletexcitation energy into singlet excitation energy (i.e., reverseintersystem crossing) using a small amount of thermal energy andefficiently generate a singlet excited state. In addition, the tripletexcitation energy can be converted into light emission.

An exciplex whose excited state is formed of two kinds of substances hasan extremely small difference between the S1 level and the T1 level andfunctions as a TADF material capable of converting triplet excitationenergy into singlet excitation energy.

A phosphorescent spectrum observed at low temperatures (e.g., 77 K to 10K) is used for an index of the T1 level. When the level of energy with awavelength of the line obtained by extrapolating a tangent to thefluorescent spectrum at a tail on the short wavelength side is the S1level and the level of energy with a wavelength of the line obtained byextrapolating a tangent to the phosphorescent spectrum at a tail on theshort wavelength side is the T1 level, the difference between the S1level and the T1 level of the TADF material is preferably less than orequal to 0.3 eV, further preferably less than or equal to 0.2 eV.

When the TADF material is used as an emission substance, the S1 level ofthe host material is preferably higher than that of the TADF material,and the T1 level of the host material is preferably higher than that ofthe TADF material.

As the host material in the light-emitting layer, variouscarrier-transport materials such as a material having anelectron-transport property, a material having a hole-transportproperty, and the TADF material can be used.

As the material having a hole-transport property, an organic compoundhaving an amine skeleton or a π-electron rich heteroaromatic ringskeleton is preferably used. Examples of the material having ahole-transport property include compounds having an aromatic amineskeleton, such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB),N,N-bis(3-methylphenyl)-N,N-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBAIBP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBilBP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), andN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); compounds having a carbazole skeleton, such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CTP), and3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); compounds having athiophene skeleton, such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), and4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-V); and compounds having a furan skeleton, such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran)(abbreviation: DBF3P-II)and 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, the compoundhaving an aromatic amine skeleton and the compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties to contribute to a reduction indriving voltage. In addition, the organic compounds given as examples ofthe above first substance can be used.

As the material having an electron-transport property, for example, ametal complex such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II)(abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAIq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiaolyl)phenolato]zinc(II) (abbreviation: ZnBTZ) or anorganic compound having a π-electron deficient heteroaromatic ringskeleton is preferably used. Examples of the organic compound having aπ-electron deficient heteroaromatic ring skeleton include a heterocycliccompound having a polyazole skeleton, such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), and2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), and4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); a heterocyclic compound having a triazine skeleton,such as2-[3′-(9,9-dimethyl-9H-fluorene-2-yl)-1,1′-biphenyl-3-yl]-4,6-diphenyl-1,3,5-triazine(abbreviation: mFBPTzn),2-[(1,1′-biphenyl)-4-yl]-4-phenyl-6-[9,9′-spirobi(9H-fluoren)-2-yl]-1,3,5-triazine(abbreviation: BP-SFTzn),2-{3-[3-(benzo[b]naphtho[1,2-d]furan-8-yl)phenyl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: mBnfBPTzn), and2-{3-[3-(benzo[b]naphtho[1,2-d]furan-6-yl)phenyl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: mBnfBPTzn-02); and a heterocyclic compound having apyridine skeleton, such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine(abbreviation: 35DCzPPy) and 1,3,5-tri[3-(3-pyridyl)phenyl]benzene(abbreviation: TmPyPB). Among the above materials, the heterocycliccompound having a diazine skeleton, the heterocyclic compound having atriazine skeleton, and the heterocyclic compound having a pyridineskeleton have high reliability and thus are preferable. In particular,the heterocyclic compound having a diazine (pyrimidine or pyrazine)skeleton and the heterocyclic compound having a triazine skeleton havean excellent electron-transport property to contribute to a reduction indriving voltage.

As the TADF material that can be used as the host material, theabove-mentioned TADF materials can also be used. When the TADF materialis used as the host material, triplet excitation energy generated in theTADF material is converted into singlet excitation energy by reverseintersystem crossing and transferred to the emission substance, wherebythe emission efficiency of the light-emitting device can be increased.Here, the TADF material functions as an energy donor, and the emissionsubstance functions as an energy acceptor.

This is very effective in the case where the emission substance is afluorescent substance, in that case, the S1 level of the TADF materialis preferably higher than the S1 level of the fluorescent substance inorder that high emission efficiency can be achieved. Furthermore, the T1level of the TADF material is preferably higher than the S1 level of thefluorescent substance. Therefore, the T1 level of the TADF material ispreferably higher than the T1 level of the fluorescent substance.

A TADF material that emits light whose wavelength overlaps with thewavelength on a lowest-energy-side absorption band of the fluorescentsubstance is preferably used, in which case excitation energy istransferred smoothly from the TADF material to the fluorescent substanceand light emission can be obtained efficiently.

In addition, in order that singlet excitation energy can be efficientlygenerated from the triplet excitation energy by reverse intersystemcrossing, carrier recombination preferably occurs in the TADF material.It is also preferable that the triplet excitation energy generated inthe TADF material not be transferred to the triplet excitation energy ofthe fluorescent substance. For that reason, the fluorescent substancepreferably has a protective group around a luminophore (a skeleton whichcauses light emission) of the fluorescent substance. As the protectivegroup, a substituent having no r bond and a saturated hydrocarbon arepreferably used. Specific examples include an alkyl group having 3 to 10carbon atoms, a substituted or unsubstituted cycloalkyl group having 3to 10 carbon atoms, and a trialkylsilyl group having 3 to 10 carbonatoms. It is further preferable that the fluorescent substance have aplurality of protective groups. The substituent having no π bond has apoor carrier-transport property, whereby the TADF material and theluminophore of the fluorescent substance can be made away from eachother with little influence on carrier transportation or carrierrecombination. Here, the luminophore refers to an atomic group(skeleton) that causes light emission in a fluorescent substance. Theluminophore is preferably a skeleton having a n bond, further preferablyincludes an aromatic ring, and still further preferably includes acondensed aromatic ring or a condensed heteroaromatic ring. Examples ofthe condensed aromatic ring or the condensed heteroaromatic ring includea phenanthrene skeleton, a stilbene skeleton, an acridone skeleton, aphenoxazine skeleton, and a phenothiazine skeleton. Specifically, afluorescent substance having any of a naphthalene skeleton, ananthracene skeleton, a fluorene skeleton, a chrysene skeleton, atriphenylene skeleton, a tetracene skeleton, a pyrene skeleton, aperylene skeleton, a coumarin skeleton, a quinacridone skeleton, and anaphthobisbenzofuran skeleton is preferred because of its highfluorescence quantum yield.

In the case where a fluorescent substance is used as the emissionsubstance, a material having an anthracene skeleton is suitably used asthe host material. The use of a substance having an anthracene skeletonas the host material for the fluorescent substance makes it possible toobtain a light-emitting layer with high emission efficiency and highdurability. Among the substances having an anthracene skeleton, asubstance having a diphenylanthracene skeleton, in particular, asubstance having a 9,10-diphenylanthracene skeleton, is chemicallystable and thus is preferably used as the host material. The hostmaterial preferably has a carbazole skeleton because the hole-injectionand hole-transport properties are improved; further preferably, the hostmaterial has a benzocarbazole skeleton in which a benzene ring isfurther condensed to carbazole because the HOMO level thereof isshallower than that of carbazole by approximately 0.1 eV and thus holesenter the host material easily. In particular, the host materialpreferably has a dibenzocarbazole skeleton because the HOMO levelthereof is shallower than that of carbazole by approximately 0.1 eV sothat holes enter the host material easily, the hole-transport propertyis improved, and the heat resistance is increased. Accordingly, asubstance that has both a 9,10-diphenylanthracene skeleton and acarbazole skeleton (or a benzocarbazole or dibenzocarbazole skeleton) isfurther preferable as the host material. Note that in terms of thehole-injection and hole-transport properties described above, instead ofa carbazole skeleton, a benzofluorene skeleton or a dibenzofluoreneskeleton may be used. Examples of such a substance include9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation:PCPN), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation:CzPA), 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA),6-[3-(9,10-diphenyl-2-anthryl)phenyl-benzo[b]naphtho(1,2-d]furan(abbreviation: 2mBnfPPA),9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4′-yl}anthracene(abbreviation: FLPPA), and9-(1-naphthyl)-10-[4-(2-naphthyl)phenyl]anthracene (abbreviation:αN-βNPAnth). Note that CzPA, cgDBCzPA, 2mBnfPPA, and PCzPA arepreferably selected because of their excellent characteristics.

Note that the host material may be a mixture of a plurality of kinds ofsubstances; in the case of using a mixed host material, it is preferableto mix an electron-transport material with a hole-transport material. Bymixing the electron-transport material with the hole-transport material,the transport property of the light-emitting layer 113 can be easilyadjusted and a recombination region can be easily controlled. The weightratio of the content of the hole-transport material to the content ofthe electron-transport material may be 1:19 to 19:1.

Note that a phosphorescent substance can be used as part of the mixedmaterial. When a fluorescent substance is used as the emissionsubstance, a phosphorescent substance can be used as an energy donor forsupplying excitation energy to the fluorescent substance.

An exciplex may be formed of these mixed materials. When these mixedmaterials are selected so as to form an exciplex that exhibits lightemission whose wavelength overlaps with the wavelength on alowest-energy-side absorption band of the emission substance, energy canbe transferred smoothly and light emission can be obtained efficiently.The use of such a structure is preferred because the driving voltage canalso be reduced.

Note that at least one of the materials forming an exciplex may be aphosphorescent substance. In this case, triplet excitation energy can beefficiently converted into singlet excitation energy by reverseintersystem crossing.

Combination of an electron-transport material and a hole-transportmaterial whose HOMO level is higher than or equal to the HOMO level ofthe electron-transport material is preferable for forming an exciplexefficiently. In addition, the LUMO level of the hole-transport materialis preferably higher than or equal to the LUMO level of theelectron-transport material. Note that the LUMO levels and the HOMOlevels of the materials can be derived from the electrochemicalcharacteristics (the reduction potentials and the oxidation potentials)of the materials that are measured by cyclic voltammetry (CV).

The formation of an exciplex can be confirmed by a phenomenon in whichthe emission spectrum of the mixed film in which the hole-transportmaterial and the electron-transport material are mixed is shifted to thelonger wavelength side than the emission spectrum of each of thematerials (or has another peak on the longer wavelength side) observedby comparison of the emission spectra of the hole-transport material,the electron-transport material, and the mixed film of these materials,for example. Alternatively, the formation of an exciplex can beconfirmed by a difference in transient response (e.g., a phenomenon inwhich the transient photoluminescence (PL) lifetime of the mixed filmhas longer lifetime components or has a larger proportion of delayedcomponents than that of each of the materials) observed by comparison oftransient PL of the hole-transport material, the electron-transportmaterial, and the mixed film of the materials. The transient PL can berephrased as transient electroluminescence (EL). That is, the formationof an exciplex can also be confirmed by a difference in transientresponse observed by comparison of the transient EL of thehole-transport material, the electron-transport material, and the mixedfilm of the materials.

The electron-transport layer 114 in the light-emitting device of oneembodiment of the present invention is formed with a material whoseresistance decreases with current flowing therethrough. In thelight-emitting device including the electron-transport layer 114 that isformed with a material whose resistance decreases with current flowingtherethrough, the resistance of the electron-transport layer 114decreases when current flows, i.e., when the light-emitting device isdriven. When the light-emitting device is designed in advance so that acarrier recombination region extends in the electron-transport layer aswell as in the light-emitting layer at the initial driving stage, thecarrier balance changes as the transport property of theelectron-transport layer 114 increases over the driving time, and theend of the recombination region on the cathode side moves toward thelight-emitting layer 113. The energy of carriers recombined in theelectron-transport layer 114 is hardly converted into light emission;thus, when the recombination region that has extended in theelectron-transport layer 114 is pushed back to the light-emitting layer113 side, the energy loss of recombination can be reduced. As a result,the emission efficiency and luminance of the light-emitting device canincrease when the light-emitting device is driven. The light-emittingdevice showing such a behavior enables a rapid decay at the initialdriving stage, which is called an initial decay, to be canceled out bythe luminance increase. Thus, the light-emitting device can have anextremely long driving lifetime owing to a smaller initial decay. Such alight-emitting device is referred to as a recombination-site tailoringinjection (ReSTI) device.

In order to fabricate the light-emitting device including a carrierrecombination region that extends in the electron-transport layer aswell as in the light-emitting layer at the initial driving stage, thehole-injection layer 111 is formed to contain the first substance, whichhas a hole-transport property and a HOMO level higher than or equal to−5.7 eV and lower than or equal to −5.4 eV, and the second substance,which exhibits an electron-accepting property with respect to the firstsubstance.

The material whose resistance decreases with current flowingtherethrough preferably contains an organometallic complex of an alkalimetal or an alkaline earth metal. The material may contain only anorganometallic complex of an alkali metal or an alkaline earth metal, ormay contain another substance in addition to the organometallic complexof an alkali metal or an alkaline earth metal. In the case where thematerial whose resistance decreases with current flowing therethroughcontains a substance other than the organometallic complex of an alkalimetal or an alkaline earth metal, the substance is preferably an organiccompound having an electron-transport property.

In the case where the material whose resistance decreases with currentflowing therethrough contains an organometallic complex of an alkalimetal or an alkaline earth metal, the organometallic complex of analkali metal or an alkaline earth metal is preferably a complex thatforms a cluster when current flows therethrough. The formation of acluster by the complex increases the electron-transport property, sothat the light-emitting device including the electron-transport layer114 with reduced resistance can be obtained.

In particular, when the material contains organometallic complexes of analkali metal or an alkaline earth metal and electron-transport organiccompounds, the complexes form a cluster and are gathered together whencurrent flows therethrough; accordingly, the electron-transport organiccompounds come close to each other. This widens the conductive pathbetween the electron-transport organic compounds to increase theelectron-transport property of the electron-transport layer 114,offering the light-emitting device including the electron-transportlayer 114 with reduced resistance.

The electron-transport organic compound preferably has anelectron-transport property that is higher than a hole-transportproperty. The electron mobility of the electron-transport organiccompound is preferably higher than or equal to 1×10⁻⁷ cm²/Vs and lowerthan or equal to 5×10⁻⁵ cm²/Vs when the square root of the electricfield strength [V/cm] is 600. The number of electrons injected into thelight-emitting layer can be controlled when the electron mobility in theelectron-transport layer is within the above range, whereby thelight-emitting layer can be prevented from having excess electrons.

The electron-transport organic compound preferably has a HOMO level of−6.0 eV or higher. The electron-transport organic compound preferablyhas an anthracene skeleton and further preferably has both an anthraceneskeleton and a heterocyclic skeleton. The heterocyclic skeleton ispreferably a nitrogen-containing five-membered ring skeleton. Inparticular, the electron-transport organic compound preferably has anitrogen-containing five-membered ring skeleton including twoheteroatoms in a ring, such as a pyrazole ring, an imidazole ring, anoxazole ring, or a thiazole ring.

The organometallic complex of an alkali metal or an alkaline earth metalis preferably a metal complex including a ligand containing nitrogen andoxygen and an alkali metal or an alkaline earth metal. In particular,the organometallic complex of an alkali metal or an alkaline earth metalis preferably a metal complex including a monovalent metal ion and aligand having an 8-hydroxyquinolinato structure, and further preferablya lithium complex including a ligand having an 8-hydroxyquinolinatostructure. Specifically, 8-hydroxyquinolinato-lithium (abbreviation:Liq), 8-hydroxyquinoline sodium salt (abbreviation: Naq), and the likecan be given, and 8-hydroxyquinolinato-lithium(abbreviation: Liq) isparticularly preferred.

Note that a mixing ratio of the electron-transport organic compound tothe organometallic complex of an alkali metal or an alkaline earth metalmay differ between regions of the electron-transport layer 114 in thethickness direction. The mixing ratio of the organometallic complex ofan alkali metal or an alkaline earth metal to the electron-transportorganic compound on the cathode side is preferably low. The mixing ratiocan be presumed by the number of atoms or molecules measured bytime-of-flight secondary ion mass spectrometry (ToF-SIMS). In regionscontaining the same two kinds of materials with different mixing ratios,the mixing ratios measured by ToF-SIMS analysis correspond to theproportions of atoms, molecules, and ions in the regions. Thus,comparison between the detected amounts of substances derived from theelectron-transport organic compound and the organometallic complex of analkali metal or an alkaline earth metal enables presumption of themixing ratio.

That is, it is preferable that the electron-transport layer 114 includethe first electron-transport layer 114-1 and the secondelectron-transport layer 114-2, the first electron-transport layer becloser to the anode side than the second electron-transport layer is,and the concentration of the organometallic complex of an alkali metalor an alkaline earth metal in the first electron-transport layer bedifferent from that in the second electron-transport layer. Note thatthe concentration of the organometallic complex of an alkali metal or analkaline earth metal is preferably higher in the firstelectron-transport layer than in the second electron-transport layer inorder that a longer lifetime device can be obtained.

When there is no clear boundary between the layers in theelectron-transport layer 114 as in FIG. 1A, the amount ratio of theelectron-transport organic compound to the organometallic complex of analkali metal or an alkaline earth metal may continuously change as inFIGS. 32A1 and 32A2. In contrast, when there is a boundary between thelayers in the electron-transport layer 114 as in FIG. 1A2, the amountratio of the electron-transport organic compound to the organometalliccomplex of an alkali metal or an alkaline earth metal may changestepwise as in FIGS. 32B1 and 32B2. Note that in the electron-transportlayer 114, a region having a high concentration of the organometalliccomplex of an alkali metal or an alkaline earth metal is preferablyprovided closer to the light-emitting layer 113 than a region having alow concentration of the organometallic complex of an alkali metal or analkaline earth metal, which is a limiting factor for theelectron-transport property. In other words, the electron-transportlayer 114 preferably includes a region where the amount (concentration)of organometallic complex of an alkali metal or an alkaline earth metalincreases from the cathode side to the anode side. Alternatively, in theelectron-transport layer 114, a region having the highest amount(concentration) of organometallic complex of an alkali metal or analkaline earth metal is preferably positioned closer to the anode sidethan a region having the lowest amount (concentration) of organometalliccomplex of an alkali metal or an alkaline earth metal is.

As the electron-transport organic compound included in theelectron-transport layer 114, any of the above-mentionedelectron-transport organic compounds that can be used as the hostmaterial, and the above-mentioned organic compounds that can be used asthe host material for the fluorescent substance can be used.

The electron mobility of the electron-transport organic compoundincluded in the electron-transport layer 114 when the square root of theelectric field strength [V/cm]is 600 is preferably lower than that ofthe host material or the light-emitting layer 113.

When the light-emitting layer has excess electrons, as illustrated inFIG. 2A, a recombination region 113-1 is limited to a part and a greatstrain is imposed on the part, which promotes degradation. In addition,electrons failing to recombine and passing through the light-emittinglayer also diminish a lifetime and emission efficiency. In oneembodiment of the present invention, a reduction in theelectron-transport property of the electron-transport layer 114 expandsthe recombination region 113-1 as illustrated in FIG. 2B and spreads thestrain on the material included in the light-emitting layer 113. Thus, alight-emitting device having a long lifetime and high emissionefficiency can be provided.

The luminance decay curve of a light-emitting device having such astructure, which is obtained by a driving test at a constant currentdensity, sometimes has the maximum value. In other words, the decaycurve of the light-emitting device of one embodiment of the presentinvention may have a portion where the luminance increases with time.The light-emitting device showing such a degradation behavior enables arapid decay at the initial driving stage, which is called an initialdecay, to be canceled out by the luminance increase. Thus, thelight-emitting device can have an extremely long driving lifetime with asmaller initial decay. Such a light-emitting device is referred to as arecombination-site tailoring injection (ReSTI) device.

A differential value of such a decay curve having the maximum value is 0in a part. Thus, the light-emitting device of one embodiment of thepresent invention with a decay curve having a differential value of 0 ina part can have an extremely long lifetime with a smaller initial decay.

The above degradation behavior is probably caused, as illustrated inFIG. 3A, by recombination that does not contribute to light emission,which occurs in a non-light-emitting recombination region 120 at theinitial driving stage because of a low electron mobility in theelectron-transport layer. That is, in the light-emitting device of thepresent invention having the above-described structure, at the initialdriving stage, a hole-injection barrier is small (the first substancehas a deep HOMO level) and the electron-transport property of theelectron-transport layer 114 is relatively low; accordingly, therecombination region 113-1 is formed extending from the light-emittinglayer 113 to the electron-transport layer 114. In addition, when theelectron-transport organic compound contained in the electron-transportlayer 114 has a relatively high HOMO level of −6.0 eV or higher, holesare likely to reach the electron-transport layer 114 because the HOMOlevel of the organometallic complex of an alkali metal or an alkalineearth metal is also −6.0 eV or higher hence, recombination is causedalso in the electron-transport layer 114, so that the non-light-emittingrecombination region 120 is easily formed.

In the light-emitting device of one embodiment of the present invention,the carrier balance changes over the driving time, and the recombinationregion 113-1 on the cathode side moves toward the hole-transport layer112 side as shown in FIG. 3B. This results in a reduction in the area ofthe non-light-emitting recombination region 120, allowing energy ofrecombined carriers to contribute to light emission effectively, so thatthe luminance increases as compared with that at the initial drivingstage. This luminance increase cancels out the rapid luminance decreaseat the initial driving stage, which is called the initial decay, of thelight-emitting device. Thus, the light-emitting device can have a longdriving lifetime with a smaller initial decay.

The present inventors have verified by experiment that a change in thecarrier balance in the light-emitting device of one embodiment of thepresent invention is caused by a change in the resistance of thelight-emitting device, in particular, a change in the resistance of theelectron-transport layer 114. The change in resistance was measured byimpedance spectroscopy (IS) measurement.

In the impedance spectroscopy measurement, a micro sinusoidal voltagesignal (V=V₀[exp(iωt)]) is applied to the light-emitting device, and theimpedance (Z=V/I) can be obtained from a phase difference between thecurrent amplitude of a response current signal (I=I₀exp[i(ωt+ϕ)]) andthe input signal.

The obtained impedance is plotted on the complex plane (a Nyquist plot)with the frequency of the applied voltage signal used as a parameter.Admittance (Y), modulus (M), dielectric constant (ε), and the like,which are basic transfer functions, can be obtained from the impedance(Z). The relationship between the basic transfer functions is asfollows.

TABLE 1 Z Y M ε Z Z 1/Y M/jω 1/jωε Y 1/Z Y jω/M jωε M jωZ jω/Y M 1/ε ε1/jωZ Y/jω 1/M ε

In this embodiment, a light-emitting device was analyzed using animpedance (Z) plot, the real axis of which gives a resistance, and amodulus (M) plot, which gives the reciprocal number of a capacitance.Table 2 shows the device structure of the light-emitting devicesubjected to the measurement.

TABLE 2 Hole- Electron- injection Hole-transport layerElectron-transport layer injection layer 1 2 Light-emitting layer 1 2layer 10 nm 20 nm 10 nm 25 nm 12.5 nm 12.5 nm 1 nm BBABnf:ALD- BBABnfPCzN2 αN-βNPAnth:3, ZADN:Liq Liq MP001Q 10PCANbf(IV)-02 (0.7:1) (1:0.7)(1:0.1) (1:0.015)

FIG. 16 shows an example of the Z plot of the light-emitting device ofone embodiment of the present invention. From the real axis of the Zplot, which represents the resistance, the resistance after drivingsignificantly decreases compared with the resistance before driving. Thelight-emitting device of one embodiment of the present invention wasthus found that the resistance largely changes before and after drivingand the resistance after driving decreases compared with that beforedriving.

The M plot of the same light-emitting device is shown in FIG. 17. Thisgraph was fitted using equivalent circuit analysis software, ZView(Scribner Associates Inc., US), offering an equivalent circuit of thelight-emitting device, which consists of four RC parallel circuits andone series resistance as shown in FIG. 18. Note that the numbers in theplot represent the corresponding resistances placed in the equivalentcircuit obtained by fitting.

FIG. 19 shows the resistance values before and after driving for each ofthe resistances shown in the equivalent circuit of FIG. 18. Thisindicates that only the resistance R2 after driving is lower by onedigit or more than that before driving.

To examine which layer corresponds to each resistance, the measurementwas performed while the thickness of the layer changed. A change in theM plot and an increase in the resistance with a large thickness showedthat the resistance R2 was derived from the electron-transport layer.

The above results revealed that when the light-emitting device was fedcurrent and driven, the electron-transport layer 114 had loweredresistance and improved electron-transport property. When theelectron-transport property of the electron-transport layer 114 isimproved, the carrier balance changes as described above, so that theend of the recombination region that has extended to theelectron-transport layer moves toward the light-emitting layer, reducingthe area of the non-light-emitting recombination region 120. Thisincreases the amount of recombined carriers in the light-emitting layer,allowing the energy of recombination to contribute to light emissioneffectively. As a result, the light-emitting device of one embodiment ofthe present invention exhibits a unique behavior the luminance is highas compared with that at the initial driving stage.

As described above, the resistance of the light-emitting device of oneembodiment of the present invention after driving is lower than thatbefore driving and the resistance of the electron-transport layer isreduced when the light-emitting device is driven. This indicates thatthe electron-transport layer is made with a material whose resistivitydecreases with current flowing therethrough, i.e., by driving thelight-emitting device.

The light-emitting device of one embodiment of the present inventionhaving the above-described structure can have an extremely longlifetime. In particular, a lifetime until when the luminance decreasesto approximately 95% of the initial luminance (LT95), which means thatthe decay is extremely small, can be significantly extended.

When the initial decay can be reduced, the problem of burn-in, which hasstill been mentioned as a great drawback of organic EL devices, and thetime and effort for aging for reducing the problem before shipment canbe significantly reduced.

Embodiment 2

Next, examples of specific structures and materials of theaforementioned light-emitting device are described. As described above,the light-emitting device of one embodiment of the present inventionincludes the EL layer 103 that is positioned between the pair ofelectrodes (the anode 101 and the cathode 102) and has a plurality oflayers. In the EL layer 103, at least the hole-injection layer 111, thefirst hole-transport layer 112-1, the second hole-transport layer 112-2,the light-emitting layer 113, and the electron-transport layer 114 areprovided from the anode 101 side.

There is no particular limitation on the other layers included in the ELlayer 103, and various layers such as a hole-injection layer, ahole-transport layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, an exciton-blocking layer, and acharge-generation layer can be employed.

The anode 101 is preferably formed using a metal, an alloy, or aconductive compound with a high work function (specifically, a workfunction of 4.0 eV or higher), a mixture thereof, or the like. Specificexamples include indium oxide-tin oxide (ITO: indium tin oxide), indiumoxide-tin oxide containing silicon or silicon oxide, indium oxide-zincoxide, and indium oxide containing tungsten oxide and zinc oxide (IWZO).Such conductive metal oxide films are usually formed by a sputteringmethod but may be formed by application of a sol-gel method or the like.In an example of the formation method, indium oxide-zinc oxide isdeposited by a sputtering method using a target obtained by adding 1 wt% to 20 wt % of zinc oxide to indium oxide. Furthermore, indium oxidecontaining tungsten oxide and zinc oxide (IWZO) can be deposited by asputtering method using a target in which tungsten oxide and zinc oxideare added to indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %,respectively. Alternatively, gold (Au), platinum (Pt), nickel (Ni),tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co),copper (Cu), palladium (Pd), nitride of a metal material (e.g., titaniumnitride), or the like can be used. Graphene can also be used. Note thatalthough the typical substances for forming the anode are listed above,a composite material of an organic compound having a hole-transportproperty and a substance exhibiting an electron-accepting property withrespect to the organic compound is used for the hole-injection layer 111of one embodiment of the present invention; thus, an electrode materialcan be selected regardless of its work function.

Two kinds of stacked layer structure of the EL layer 103 are described:structures illustrated in FIGS. 1A1 and 1A2, each of which includes theelectron-injection layer 115 in addition to the hole-injection layer111, the first hole-transport layer 112-1, the second hole-transportlayer 112-2, the light-emitting layer 113, and the electron-transportlayer 114 (the first electron-transport layer 114-1 and the secondelectron-transport layer 114-2); and a structure illustrated in FIG. 18,which includes a charge-generation layer 116 instead of theelectron-injection layer 115. Materials for the layers will bespecifically described below.

Since the hole-injection layer 111, the hole-transport layer 112 (thefirst hole-transport layer 112-1 and the second hole-transport layer112-2), the light-emitting layer 113, and the electron-transport layer114 (the first electron-transport layer 114-1 and the secondelectron-transport layer 114-2) are described in detail in Embodiment 1,the description thereof is not repeated. Refer to the description inEmbodiment 1.

A layer containing an alkali metal, an alkaline earth metal, or acompound thereof such as lithium fluoride (LiF), cesium fluoride (CsF),or calcium fluoride (CaF₂) may be provided as the electron-injectionlayer 115 between the electron-transport layer 114 and the cathode 102.For example, an electrode or a layer that is formed using a substancehaving an electron-transport property and that contains an alkali metal,an alkaline earth metal, or a compound thereof may be used as theelectron-injection layer 115. Examples of the electrode include asubstance in which electrons are added at high concentration to calciumoxide-aluminum oxide.

Instead of the electron-injection layer 115, the charge-generation layer116 may be provided between the electron-transport layer 114 and thecathode 102 (FIG. 1B). The charge-generation layer 116 refers to a layercapable of injecting holes into a layer in contact with the cathode sideof the charge-generation layer 116 and electrons into a layer in contactwith the anode side thereof when a potential is applied. Thecharge-generation layer 116 includes at least a p-type layer 117. Thep-type layer 117 is preferably formed using any of the compositematerials given above as examples of the material that can be used forthe hole-injection layer 111. The p-type layer 117 may be formed bystacking a film containing the above-described acceptor material as amaterial included in the composite material and a film containing ahole-transport material. When a potential is applied to the p-type layer117, electrons are injected into the electron-transport layer 114 andholes are injected into the cathode 102; thus, the light-emitting deviceoperates.

Note that the charge-generation layer 116 preferably includes anelectron-relay layer 118 and/or an electron-injection buffer layer 119in addition to the p-type layer 117.

The electron-relay layer 118 contains at least the substance having anelectron-transport property and has a function of preventing aninteraction between the electron-injection buffer layer 119 and thep-type layer 117 and smoothly transferring electrons. The LUMO level ofthe substance having an electron-transport property contained in theelectron-relay layer 118 is preferably between the LUMO level of theelectron-accepting substance in the p-type layer 117 and the LUMO levelof a substance contained in a layer of the electron-transport layer 114that is in contact with the charge-generation layer 116. As a specificvalue of the energy level, the LUMO level of the substance having anelectron-transport property in the electron-relay layer 118 ispreferably higher than or equal to −5.0 eV, further preferably higherthan or equal to −5.0 eV and lower than or equal to −3.0 eV. Note thatas the substance having an electron-transport property in theelectron-relay layer 118, a phthalocyanine-based material or a metalcomplex having a metal-oxygen bond and an aromatic ligand is preferablyused.

A substance having an excellent electron-injection property can be usedfor the electron-injection buffer layer 119. For example, an alkalimetal, an alkaline earth metal, a rare earth metal, or a compoundthereof (an alkali metal compound (including an oxide such as lithiumoxide, a halide, and a carbonate such as lithium carbonate and cesiumcarbonate), an alkaline earth metal compound (including an oxide, ahalide, and a carbonate), or a rare earth metal compound (including anoxide, a halide, and a carbonate)) can be used.

In the case where the electron-injection buffer layer 119 contains thesubstance having an electron-transport property and a substance havingan electron-donating property, an organic compound such astetrathianaphthacene (abbreviation: TTN), nickelocene, ordecamethylnickelocene, as well as an alkali metal, an alkaline earthmetal, a rare earth metal, or a compound thereof (an alkali metalcompound (including an oxide such as lithium oxide, a halide, and acarbonate such as lithium carbonate and cesium carbonate), an alkalineearth metal compound (including an oxide, a halide, and a carbonate), ora rare earth metal compound (including an oxide, a halide, and acarbonate)), can be used as the substance having an electron-donatingproperty. As the substance having an electron-transport property, amaterial similar to the above-described material for theelectron-transport layer 114 can be used.

For the cathode 102, a metal, an alloy, or an electrically conductivecompound with a low work function (specifically, a work function of 3.8eV or lower), a mixture thereof, or the like can be used. Specificexamples of such a cathode material include elements belonging to Group1 or 2 of the periodic table, such as alkali metals (e.g. lithium (Li)and cesium(Cs)), magnesium (Mg), calcium (Ca), and strontium (Sr),alloys containing these elements (e.g., MgAg and AlLi), rare earthmetals such as europium (Eu) and ytterbium (Yb), and alloys containingthese rare earth metals. However, when the electron-injection layer isprovided between the cathode 102 and the electron-transport layer, avariety of conductive materials such as Al, Ag, ITO, or indium oxide-tinoxide containing silicon or silicon oxide can be used for the cathode102 regardless of the work function. Films of these conductive materialscan be formed by a dry process such as a vacuum evaporation method or asputtering method, an inkjet method, a spin coating method, or the like.Alternatively, a wet process using a sol-gel method or a wet processusing a paste of a metal material may be employed.

Furthermore, any of a variety of methods can be used for forming the ELlayer 103, regardless of whether it is a dry process or a wet process.For example, a vacuum evaporation method, a gravure printing method, anoffset printing method, a screen printing method, an inkjet method, or aspin coating method may be used.

Different methods may be used to form the electrodes or the layersdescribed above.

The structure of the layers provided between the anode 101 and thecathode 102 is not limited to the above-described structure. Preferably,alight-emitting region where holes and electrons recombine is positionedaway from the anode 101 and the cathode 102 so that quenching due to theproximity of the light-emitting region and a metal used for electrodesand carrier-injection layers can be prevented.

Furthermore, in order that transfer of energy from an exciton generatedin the light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer that are incontact with the light-emitting layer 113, particularly acarrier-transport layer closer to the recombination region in thelight-emitting layer 113, are formed using a substance having a widerband gap than the light-emitting substance of the light-emitting layeror the light-emitting substance included in the light-emitting layer.

Next, an embodiment of a light-emitting device with a structure in whicha plurality of light-emitting units are stacked (this type oflight-emitting device is also referred to as a stacked or tandemlight-emitting device) is described with reference to FIG. 1C. Thislight-emitting device includes a plurality of light-emitting unitsbetween an anode and a cathode. One light-emitting unit hassubstantially the same structure as the EL layer 103 illustrated in FIG.1A1 or FIG. 1A2. In other words, the light-emitting device illustratedin FIG. 1A1, FIG. 1A2, or FIG. 1B includes a single light-emitting unitwhereas the light-emitting device illustrated in FIG. 1C includes aplurality of light-emitting units.

In FIG. 1C, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between an anode 501 and a cathode 502, and acharge-generation layer 513 is provided between the first light-emittingunit 511 and the second light-emitting unit 512. The anode 501 and thecathode 502 correspond, respectively, to the anode 101 and the cathode102 illustrated in FIG. 1A1, and the materials given in the descriptionfor FIG. 1A1 can be used. Furthermore, the first light-emitting unit 511and the second light-emitting unit 512 may have the same structure ordifferent structures.

The charge-generation layer 513 has a function of injecting electronsinto one of the light-emitting units and injecting holes into the otherof the light-emitting units when a voltage is applied between the anode501 and the cathode 502. That is, in FIG. 1C, the charge-generationlayer 513 injects electrons into the first light-emitting unit 511 andholes into the second light-emitting unit 512 when a voltage is appliedso that the potential of the anode becomes higher than the potential ofthe cathode.

The charge-generation layer 513 preferably has a structure similar tothat of the charge-generation layer 116 described with reference to FIG.1B. A composite material of an organic compound and a metal oxide has anexcellent carrier-injection property and an excellent carrier-transportproperty; thus, low-voltage driving and low-current driving can beachieved. In the case where the anode-side surface of a light-emittingunit is in contact with the charge-generation layer 513, thecharge-generation layer 513 can also function as a hole-injection layerof the light-emitting unit; therefore, a hole-injection layer is notnecessarily provided in the light-emitting unit.

In the case where the charge-generation layer 513 includes theelectron-injection buffer layer 119, the electron-injection buffer layer119 functions as an electron-injection layer in the light-emitting uniton the anode side; thus, an electron-injection layer is not necessarilyformed in the light-emitting unit on the anode side.

The light-emitting device having two light-emitting units is describedwith reference to FIG. 1C; however, one embodiment of the presentinvention can also be applied to a light-emitting device in which threeor more light-emitting units are stacked. With a plurality oflight-emitting units partitioned by the charge-generation layer 513between a pair of electrodes as in the light-emitting device of thisembodiment, it is possible to provide a long-life device that can emitlight with high luminance at a low current density. A light-emittingapparatus that can be driven at a low voltage and has low powerconsumption can also be provided.

When the emission colors of the light-emitting units are different,light emission of a desired color can be obtained from thelight-emitting device as a whole. For example, in a light-emittingdevice having two light-emitting units, the emission colors of the firstlight-emitting unit may be red and green and the emission color of thesecond light-emitting unit may be blue, so that the light-emittingdevice can emit white light as a whole. The light-emitting device inwhich three or more light-emitting units are stacked can be, forexample, a tandem device in which a first light-emitting unit includes afirst blue light-emitting layer, a second light-emitting unit includes ayellow or yellow-green light-emitting layer and a red light-emittinglayer, and a third light-emitting unit includes a second bluelight-emitting layer. The tandem device can provide white light emissionlike the above light-emitting device.

The above-described layers and electrodes such as the EL layer 103, thefirst light-emitting unit 511, the second light-emitting unit 512, andthe charge-generation layer can be formed by a method such as anevaporation method (including a vacuum evaporation method), a dropletdischarge method (also referred to as an inkjet method), a coatingmethod, or a gravure printing method. A low molecular material, a middlemolecular material (including an oligomer and a dendrimer), or a highmolecular material may be included in the layers and electrodes.

Embodiment 3

In this embodiment, a light-emitting apparatus including thelight-emitting device described in Embodiments 1 and 2 will bedescribed.

In this embodiment, the light-emitting apparatus manufactured using thelight-emitting device described in Embodiments 1 and 2 is described withreference to FIGS. 4A and 4B. Note that FIG. 4A is atop view of thelight-emitting apparatus and FIG. 4B is a cross-sectional view takenalong the lines A-B and C-D in FIG. 4A. This light-emitting apparatusincludes a driver circuit portion (source line driver circuit) 601, apixel portion 602, and a driver circuit portion (gate line drivercircuit) 603, which control light emission of a light-emitting deviceand are illustrated with dotted lines. A reference numeral 604 denotes asealing substrate, a reference numeral 605 denotes a sealant, and areference numeral 607 denotes a space surrounded by the sealant 605.

A lead wiring 608 is a wiring for transmitting signals to be input tothe source line driver circuit 601 and the gate line driver circuit 603and receiving signals such as a video signal, a clock signal, a startsignal, and a reset signal from a flexible printed circuit (FPC) 609serving as an external input terminal. Although only the FPC isillustrated here, a printed wiring board (PWB) may be attached to theFPC. The light-emitting apparatus in this specification includes, in itscategory, not only the light-emitting apparatus itself but also thelight-emitting apparatus provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.4B. The driver circuit portions and the pixel portion are formed over anelement substrate 610. Here, the source line driver circuit 601, whichis a driver circuit portion, and one pixel in the pixel portion 602 areillustrated.

The element substrate 610 may be a substrate containing glass, quartz,an organic resin, a metal, an alloy, or a semiconductor or a plasticsubstrate formed of fiber reinforced plastics (FRP), polyvinyl fluoride(PVF), polyester, acrylic, or the like.

The structure of transistors used in pixels and driver circuits is notparticularly limited. For example, inverted staggered transistors may beused, or staggered transistors may be used. Furthermore, top-gatetransistors or bottom-gate transistors may be used. A semiconductormaterial used for the transistors is not particularly limited, and forexample, silicon, germanium, silicon carbide, gallium nitride, or thelike can be used. Alternatively, an oxide semiconductor containing atleast one of indium, gallium, and zinc, such as an In—Ga—Zn-based metaloxide, may be used.

There is no particular limitation on the crystallinity of asemiconductor material used for the transistors, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. A semiconductor having crystallinity ispreferably used, in which case degradation of the transistorcharacteristics can be suppressed.

Here, an oxide semiconductor is preferably used for semiconductordevices such as the transistors provided in the pixels and drivercircuits and transistors used for touch sensors described later, and thelike. In particular, an oxide semiconductor having a wider band gap thansilicon is preferably used. When an oxide semiconductor having a widerband gap than silicon is used, the off-state current of the transistorscan be reduced.

The oxide semiconductor preferably contains at least indium (In) or zinc(Zn). Further preferably, the oxide semiconductor contains an oxiderepresented by an In-M-Zn-based oxide (M represents a metal such as Al,Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf).

An oxide semiconductor that can be used in one embodiment of the presentinvention is described below.

Oxide semiconductors are classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

The CAAC-OS has c-axis alignment, its nanocrystals are connected in thea-b plane direction, and its crystal structure has distortion. Note thatdistortion refers to a portion where the direction of a latticearrangement changes between a region with a uniform lattice arrangementand another region with a uniform lattice arrangement in a region wherethe nanocrystals are connected.

The shape of the nanocrystal is basically a hexagon but is not always aregular hexagon and is a non-regular hexagon in some cases. A pentagonallattice arrangement, a heptagonal lattice arrangement, and the like areincluded in the distortion in some cases. Note that it is difficult toobserve a clear grain boundary even in the vicinity of distortion in theCAAC-OS. That is, a lattice arrangement is distorted and thus formationof a grain boundary is inhibited. This is because the CAAC-OS cantolerate distortion owing to a low density of oxygen atom arrangement inthe a-b plane direction, a change in interatomic bond distance bysubstitution of a metal element, and the like.

The CAAC-OS tends to have a layered crystal structure (also referred toas a stacked-layer structure) in which a layer containing indium andoxygen (hereinafter, an In layer) and a layer containing the element M,zinc, and oxygen (hereinafter, an (M, Zn) layer) are stacked. Note thatindium and the element M can be replaced with each other, and when theelement M of the (M, Zn) layer is replaced with indium, the layer can bereferred to as an (In, M, Zn) layer. When indium of the In layer isreplaced with the element M, the layer can be referred to as an (In, M)layer.

The CAAC-OS is an oxide semiconductor with high crystallinity. Bycontrast, in the CAAC-OS, a reduction in electron mobility due to agrain boundary is less likely to occur because it is difficult toobserve a clear grain boundary. Entry of impurities, formation ofdefects, or the like might decrease the crystallinity of an oxidesemiconductor. This means that the CAAC-OS is an oxide semiconductorhaving small amounts of impurities and defects (e.g., oxygen vacancies(V_(O))). Thus, an oxide semiconductor including the CAAC-OS isphysically stable. Accordingly, the oxide semiconductor including theCAAC-OS is resistant to heat and has high reliability.

In the nc-OS, a microscopic region (e.g., a region with a size greaterthan or equal to 1 nm and less than or equal to 10 nm, in particular, aregion with a size greater than or equal to 1 nm and less than or equalto 3 nm) has a periodic atomic arrangement. There is no regularity ofcrystal orientation between different nanocrystals in the nc-OS. Thus,the orientation in the whole film is not observed. Accordingly, in somecases, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method.

Note that an indium-gallium-zinc oxide (hereinafter, IGZO) that is anoxide semiconductor containing indium, gallium, and zinc has a stablestructure in some cases by being formed of the above-describednanocrystals. In particular, IGZO crystals tend not to grow in the airand thus, a stable structure is obtained when IGZO is formed of smallercrystals (e.g., the above-described nanocrystals) rather than largercrystals (here, crystals with a size of several millimeters or severalcentimeters).

The a-like OS is an oxide semiconductor having a structure between thoseof the nc-OS and the amorphous oxide semiconductor. The a-like OS has avoid or a low-density region. That is, the a-like OS has lowcrystallinity as compared with the nc-OS and the CAAC-OS.

An oxide semiconductor can have any of various structures that showvarious different properties. Two or more of the amorphous oxidesemiconductor, the polycrystalline oxide semiconductor, the a-like OS,the nc-OS, and the CAAC-OS may be included in an oxide semiconductor ofone embodiment of the present invention.

A cloud-aligned composite OS (CAC-OS) may be used as an oxidesemiconductor other than the above.

A CAC-OS has a conducting function in part of the material and has aninsulating function in another part of the material; as a whole, theCAC-OS has a function of a semiconductor. Note that in the case wherethe CAC-OS is used in a semiconductor layer of a transistor, theconducting function is to allow electrons (or holes) serving as carriersto flow, and the insulating function is to not allow electrons servingas carriers to flow. By the complementary action of the conductingfunction and the insulating function, the CAC-OS can have a switchingfunction (on/off function). In the CAC-OS, separation of the functionscan maximize each function.

Furthermore, the CAC-OS includes conductive regions and insulatingregions. The conductive regions have the above-described conductingfunction, and the insulating regions have the above-described insulatingfunction. In some cases, the conductive regions and the insulatingregions in the material are separated at the nanoparticle level.Furthermore, in some cases, the conductive regions and the insulatingregions are unevenly distributed in the material. The conductive regionsare sometimes observed to be coupled in a cloud-like manner with theirboundaries blurred.

Furthermore, in the CAC-OS, the conductive regions and the insulatingregions each have a size greater than or equal to 0.5 nm and less thanor equal to 10 nm, preferably greater than or equal to 0.5 nm and lessthan or equal to 3 nm, and are dispersed in the material, in some cases.

The CAC-OS includes components having different band gaps. For example,the CAC-OS includes a component having a wide gap due to the insulatingregion and a component having a narrow gap due to the conductive region.In the case of such a composition, carriers mainly flow in the componenthaving a narrow gap. The component having a narrow gap complements thecomponent having a wide gap, and carriers also flow in the componenthaving a wide gap in conjunction with the component having a narrow gap.Therefore, in the case where the above-described CAC-OS is used in achannel formation region of a transistor, high current drive capabilityin the on state of the transistor, that is, high on-state current andhigh field-effect mobility, can be obtained.

In other words, the CAC-OS can also be referred to as a matrix compositeor a metal matrix composite.

The use of the above-described oxide semiconductor materials for thesemiconductor layer makes it possible to provide a highly reliabletransistor in which a change in the electrical characteristics issuppressed.

Charge accumulated in a capacitor through a transistor including theabove-described semiconductor layer can be held for a long time becauseof the low off-state current of the transistor. When such a transistoris used in a pixel, operation of a driver circuit can be stopped while agray scale of an image displayed on each display region is maintained.As a result, an electronic device with extremely low power consumptioncan be obtained.

For stable characteristics or the like of the transistor, a base film ispreferably provided. The base film can be formed with a single-layerstructure or a stacked-layer structure using an inorganic insulatingfilm such as a silicon oxide film, a silicon nitride film, a siliconoxynitride film, or a silicon nitride oxide film. The base film can beformed by a sputtering method, a chemical vapor deposition (CVD) method(e.g., a plasma CVD method, a thermal CVD method, or a metal organic CVD(MOCVD) method), an atomic layer deposition (ALD) method, a coatingmethod, a printing method, or the like. Note that the base film is notnecessarily provided.

Note that an FET 623 is illustrated as a transistor formed in the drivercircuit portion 601. In addition, the driver circuit may be formed withany of a variety of circuits such as a CMOS circuit, a PMOS circuit, andan NMOS circuit. Although a driver integrated type in which the drivercircuit is formed over the substrate is illustrated in this embodiment,the driver circuit is not necessarily formed over the substrate, and thedriver circuit can be formed outside the substrate.

The pixel portion 602 includes a plurality of pixels each including aswitching FET 611, a current controlling FET 612, and an anode 613electrically connected to a drain of the current controlling FET 612.One embodiment of the present invention is not limited to the structure.The pixel portion 602 may include three or more FETs and a capacitor incombination.

Note that to cover an end portion of the anode 613, an insulator 614 isformed. Here, the insulator 614 can be formed using positivephotosensitive acrylic.

In order to improve the coverage with an EL layer or the like which isformed later, the insulator 614 is formed to have a curved surface withcurvature at its upper or lower end portion. For example, in the casewhere positive photosensitive acrylic is used as a material of theinsulator 614, only the upper end portion of the insulator 614preferably has a curved surface with a curvature radius (0.2 μm to 3μm). As the insulator 614, either a negative photosensitive resin or apositive photosensitive resin can be used.

An EL layer 616 and a cathode 617 are formed over the anode 613. Here,as a material used for the anode 613, a material having a high workfunction is desirably used. For example, a single-layer film of an ITOfilm, an indium tin oxide film containing silicon, an indium oxide filmcontaining zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like, astack of a titanium nitride film and a film containing aluminum as itsmain component, a stack of three layers of a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, or the like can be used. The stacked-layer structure enables lowwiring resistance and favorable ohmic contact, and can function as ananode.

The EL layer 616 is formed by any of a variety of methods such as anevaporation method using an evaporation mask, an inkjet method, and aspin coating method. The EL layer 616 has the structure described inEmbodiments 1 and 2. As another material included in the EL layer 616, alow molecular compound or a high molecular compound (including anoligomer or a dendrimer) may be used.

As a material used for the cathode 617, which is formed over the ELlayer 616, a material having a low work function (e.g., Al, Mg, Li, andCa, or an alloy or a compound thereof, such as MgAg, MgIn, or AlLi) ispreferably used. In the case where light generated in the EL layer 616is transmitted through the cathode 617, a stack of a thin metal film anda transparent conductive film (e.g., ITO, indium oxide containing zincoxide at 2 wt % to 20 wt %, indium tin oxide containing silicon, or zincoxide (ZnO)) is preferably used for the cathode 617.

Note that the light-emitting device is formed with the anode 613, the ELlayer 616, and the cathode 617. The light-emitting device is thelight-emitting device described in Embodiments 1 and 2. In thelight-emitting apparatus of this embodiment, the pixel portion, whichincludes a plurality of light-emitting devices, may include both thelight-emitting device described in Embodiments 1 and 2 and alight-emitting device having a different structure.

The sealing substrate 604 is attached to the element substrate 610 withthe sealant 605, so that a light-emitting device 618 is provided in thespace 607 surrounded by the element substrate 610, the sealing substrate604, and the sealant 605. The space 607 is filled with a filler, and maybe filled with an inert gas (such as nitrogen or argon) or the sealant.It is preferable that the sealing substrate have a recessed portionprovided with a desiccant, in which case degradation due to theinfluence of moisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealant605. It is desirable that such a material transmit moisture or oxygen aslittle as possible. As the sealing substrate 604, a glass substrate, aquartz substrate, or a plastic substrate formed of fiber reinforcedplastics (FRP), polyvinyl fluoride (PVF), polyester, or acrylic can beused.

Although not illustrated in FIGS. 4A and 4B, a protective film may beprovided over the cathode. As the protective film, an organic resin filmor an inorganic insulating film may be formed. The protective film maybe formed so as to cover an exposed portion of the sealant 605. Theprotective film can be provided so as to cover surfaces and sidesurfaces of the pair of substrates and exposed side surfaces of asealing layer, an insulating layer, and the like.

The protective film can be formed using a material through whichimpurities such as water do not permeate easily. Thus, diffusion ofimpurities such as water from the outside into the inside can beeffectively suppressed.

As a material of the protective film, an oxide, a nitride, a fluoride, asulfide, a ternary compound, a metal, a polymer, or the like can beused. For example, the material may contain aluminum oxide, hafniumoxide, hafnium silicate, lanthanum oxide, silicon oxide, strontiumtitanate, tantalum oxide, titanium oxide, zinc oxide, niobium oxide,zirconium oxide, tin oxide, yttrium oxide, cerium oxide, scandium oxide,erbium oxide, vanadium oxide, indium oxide, aluminum nitride, hafniumnitride, silicon nitride, tantalum nitride, titanium nitride, niobiumnitride, molybdenum nitride, zirconium nitride, gallium nitride, anitride containing titanium and aluminum, an oxide containing titaniumand aluminum, an oxide containing aluminum and zinc, a sulfidecontaining manganese and zinc, a sulfide containing cerium andstrontium, an oxide containing erbium and aluminum, an oxide containingyttrium and zirconium, or the like.

The protective film is preferably formed using a deposition method withfavorable step coverage. One such method is an atomic layer deposition(ALD)method. A material that can be deposited by an ALD method ispreferably used for the protective film. A dense protective film havingreduced defects such as cracks or pinholes or a uniform thickness can beformed by an ALD method. Furthermore, damage to a process member informing the protective film can be reduced.

By an ALD method, a uniform protective film with few defects can beformed even on a surface with a complex uneven shape or upper, side, andlower surfaces of a touch panel.

As described above, the light-emitting apparatus manufactured using thelight-emitting device described in Embodiments 1 and 2 can be obtained.

The light-emitting apparatus in this embodiment is manufactured usingthe light-emitting device described in Embodiments 1 and 2 and thus canhave favorable characteristics. Specifically, since the light-emittingdevice described in Embodiments 1 and 2 has along lifetime, thelight-emitting apparatus can have high reliability. Since thelight-emitting apparatus using the light-emitting device described inEmbodiments 1 and 2 has high emission efficiency, the light-emittingapparatus can achieve low power consumption.

FIGS. 5A and 5B each illustrate an example of a light-emitting apparatusthat includes a light-emitting device exhibiting white light emissionand coloring layers (color filters) and the like to display a full-colorimage. FIG. A illustrates a substrate 1001, a base insulating film 1002,a gate insulating film 1003, gate electrodes 1006, 1007, and 1008, afirst interlayer insulating film 1020, a second interlayer insulatingfilm 1021, a peripheral portion 1042, a pixel portion 1040, a drivercircuit portion 1041, anodes 1024W, 1024R, 1024G, and 1024B oflight-emitting devices, a partition 1025, an EL layer 1028, a cathode1029 of the light-emitting devices, a sealing substrate 1031, a sealant1032, and the like.

In FIG. 5A, coloring layers (a red coloring layer 1034R, a greencoloring layer 1034G, and a blue coloring layer 1034B) are provided on atransparent base material 1033. A black matrix 1035 may be additionallyprovided. The transparent base material 1033 provided with the coloringlayers and the black matrix is aligned and fixed to the substrate 1001.Note that the coloring layers and the black matrix 1035 are covered withan overcoat layer 1036. In FIG. 5A, light emitted from part of thelight-emitting layer does not pass through the coloring layers, whilelight emitted from the other part of the light-emitting layer passesthrough the coloring layers. The light that does not pass through thecoloring layers is white and the light that passes through any one ofthe coloring layers is red, green, or blue; thus, an image can bedisplayed using pixels of the four colors.

FIG. 5B illustrates an example in which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided between the gate insulating film 1003and the first interlayer insulating film 1020. As in this structure, thecoloring layers may be provided between the substrate 1001 and thesealing substrate 1031.

The above-described light-emitting apparatus has a structure in whichlight is extracted from the substrate 1001 side where FETs are formed (abottom emission structure), but may have a structure in which light isextracted from the sealing substrate 1031 side (a top emissionstructure). FIG. 6 is a cross-sectional view of a light-emittingapparatus having a top emission structure. In this case, a substratethat does not transmit light can be used as the substrate 1001. Theprocess up to the step of forming a connection electrode that connectsthe FET and the anode of the light-emitting device is performed in amanner similar to that of the light-emitting apparatus having a bottomemission structure. Then, a third interlayer insulating film 1037 isformed to cover an electrode 1022. This insulating film may have aplanarization function. The third interlayer insulating film 1037 can beformed using a material similar to that of the second interlayerinsulating film or using any of other known materials.

The anodes 1024W, 1024R, 1024G, and 1024B of the light-emitting devicesare anodes here, but may be formed as cathodes. Furthermore, in the caseof a light-emitting apparatus having a top emission structure asillustrated in FIG. 6, the anodes are preferably reflective electrodes.The EL layer 1028 is formed to have a structure similar to the structureof the EL layer 103 described in Embodiments 1 and 2, with which whitelight emission can be obtained.

In the case of a top emission structure as illustrated in FIG. 6,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the black matrix 1035 that ispositioned between pixels. The coloring layers (the red coloring layer1034R, the green coloring layer 1034G, and the blue coloring layer1034B) and the black matrix may be covered with an overcoat layer. Notethat a light-transmitting substrate is used as the sealing substrate1031. Although an example in which full color display is performed usingfour colors of red, green, blue, and white is shown here, there is noparticular limitation and full color display using four colors of red,yellow, green, and blue or three colors of red, green, and blue may beperformed.

In the light-emitting apparatus having a top emission structure, amicrocavity structure can be suitably employed. A light-emitting devicewith a microcavity structure is formed with the use of a reflectiveelectrode as the anode and a transflective electrode as the cathode. Thelight-emitting device with a microcavity structure includes at least anEL layer between the reflective electrode and the transflectiveelectrode. The EL layer includes at least a light-emitting layer servingas a light-emitting region.

Note that the reflective electrode has a visible light reflectivity of40% to 100%, preferably 70% to 100%, and a resistivity of 1×10⁻² Ωcm orlower. In addition, the transflective electrode has a visible lightreflectivity of 20% to 80%, preferably 40% to 70%, and a resistivity of1×10⁻² Ωcm or lower.

Light emitted from the light-emitting layer included in the EL layer isreflected and resonated by the reflective electrode and thetransflective electrode.

In the light-emitting device, by changing the thicknesses of thetransparent conductive film, the composite material, thecarrier-transport material, and the like, the optical path lengthbetween the reflective electrode and the transflective electrode can bechanged. Thus, light with a wavelength that is resonated between thereflective electrode and the transflective electrode can be intensifiedwhile light with a wavelength that is not resonated therebetween can beattenuated.

Note that light that is reflected back by the reflective electrode(first reflected light) considerably interferes with light that directlyenters the transflective electrode from the light-emitting layer (firstincident light). For this reason, the optical path length between thereflective electrode and the light-emitting layer is preferably adjustedto (2n−1)λ/4 (n is a natural number of 1 or larger and λ is a wavelengthof color to be amplified). By adjusting the optical path length, thephases of the first reflected light and the first incident light can bealigned with each other and the light emitted from the light-emittinglayer can be further amplified.

Note that in the above structure, the EL layer may include a pluralityof light-emitting layers or may include a single light-emitting layer.The tandem light-emitting device described above may be combined withthe EL layer; for example, a light-emitting device may have a structurein which a plurality of EL layers are provided, a charge-generationlayer is provided between the EL layers, and each EL layer includes aplurality of light-emitting layers or a single light-emitting layer.

With the microcavity structure, emission intensity with a specificwavelength in the front direction can be increased, whereby powerconsumption can be reduced. Note that in the case of a light-emittingapparatus that displays images with subpixels of four colors, red,yellow, green, and blue, the light-emitting apparatus can have favorablecharacteristics because the luminance can be increased owing to yellowlight emission and each subpixel can employ a microcavity structuresuitable for wavelengths of the corresponding color.

The light-emitting apparatus in this embodiment is manufactured usingthe light-emitting device described in Embodiments 1 and 2 and thus canhave favorable characteristics. Specifically, since the light-emittingdevice described in Embodiments 1 and 2 has a long lifetime, thelight-emitting apparatus can have high reliability. Since thelight-emitting apparatus using the light-emitting device described inEmbodiments 1 and 2 has high emission efficiency, the light-emittingapparatus can achieve low power consumption.

The active matrix light-emitting apparatus is described above, whereas apassive matrix light-emitting apparatus is described below FIGS. 7A and7B illustrate a passive matrix light-emitting apparatus manufacturedusing the present invention. Note that FIG. 7A is a perspective view ofthe light-emitting apparatus, and FIG. 7B is a cross-sectional viewtaken along the line X-Y in FIG. 7A. In FIGS. 7A and 7B, over asubstrate 951, an EL layer 955 is provided between an electrode 952 andan electrode 956. An end portion of the electrode 952 is covered with aninsulating layer 953. A partition layer 954 is provided over theinsulating layer 953. The sidewalls of the partition layer 954 areaslope such that the distance between the sidewalls is graduallynarrowed toward the surface of the substrate. In other words, a crosssection taken along the direction of the short side of the partitionlayer 954 is trapezoidal, and the lower side (a side of the trapezoidthat is parallel to the surface of the insulating layer 953 and is incontact with the insulating layer 953) is shorter than the upper side (aside of the trapezoid that is parallel to the surface of the insulatinglayer 953 and is not in contact with the insulating layer 953). Thepartition layer 954 provided in this manner can prevent defects of thelight-emitting device due to static electricity or the like. The passivematrix light-emitting apparatus also includes the light-emitting devicedescribed in Embodiments 1 and 2; thus, the light-emitting apparatus canhave high reliability or low power consumption.

Since many minute light-emitting devices arranged in a matrix in thelight-emitting apparatus described above can be independentlycontrolled, the light-emitting apparatus can be suitably used as adisplay device for displaying images.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 4

In this embodiment, an example in which the light-emitting devicedescribed in Embodiments 1 and 2 is used for a lighting device isdescribed with reference to FIGS. 8A and 8B. FIG. 8B is atop view of thelighting device, and FIG. 8A is a cross-sectional view taken along theline e-f in FIG. 8B.

In the lighting device in this embodiment, an anode 401 is formed over asubstrate 400 which is a support and has a light-transmitting property.The anode 401 corresponds to the anode 101 in Embodiment 2. When lightis extracted from the anode 401 side, the anode 401 is formed using amaterial having a light-transmitting property.

A pad 412 for applying voltage to a cathode 404 is formed over thesubstrate 400.

An EL layer 403 is formed over the anode 401. The structure of the ELlayer 403 corresponds to, for example, the structure of the EL layer 103in Embodiments 1 and 2, or the structure in which the light-emittingunits 511 and 512 and the charge-generation layer 513 are combined.Refer to the descriptions for the structures.

The cathode 404 is formed to cover the EL layer 403. The cathode 404corresponds to the cathode 102 in Embodiment 2. The cathode 404 isformed using a material having high reflectivity when light is extractedfrom the anode 401 side. The cathode 404 is connected to the pad 412,whereby voltage is applied.

As described above, the lighting device described in this embodimentincludes a light-emitting device including the anode 401, the EL layer403, and the cathode 404. Since the light-emitting device has highemission efficiency, the lighting device in this embodiment can have lowpower consumption.

The substrate 400 provided with a light-emitting device having the abovestructure is fixed to a sealing substrate 407 with sealants 405 and 406and sealing is performed, whereby the lighting device is completed. Itis possible to use only either the sealant 405 or the sealant 406. Theinner sealant 406 (not illustrated in FIG. 8B) can be mixed with adesiccant that enables moisture in a space 408 to be adsorbed, whichresults in improved reliability.

When parts of the pad 412 and the anode 401 are extended to the outsideof the sealants 405 and 406, the extended parts can function as externalinput terminals. An IC chip 420 mounted with a converter or the like maybe provided over the external input terminals.

The lighting device described in this embodiment includes, as an ELdevice, the light-emitting device described in Embodiments 1 and 2;thus, the light-emitting apparatus can have high reliability. Inaddition, the light-emitting apparatus can consume less power.

Embodiment 5

In this embodiment, examples of electronic devices each including thelight-emitting device described in Embodiments 1 and 2 are described.The light-emitting device described in Embodiments 1 and 2 has a longlifetime and high reliability. As a result, the electronic devicesdescribed in this embodiment can each include a light-emitting portionhaving high reliability.

Examples of the electronic devices including the above light-emittingdevice include a television device (also referred to as a television ora television receiver), a monitor for a computer or the like, a digitalcamera, a digital video camera, a digital photo frame, a cellular phone(also referred to as a mobile phone or a mobile phone device), aportable game machine, a portable information terminal, an audioplayback device, and a large game machine such as a pachinko machine.Specific examples of these electronic devices are described below.

FIG. 9A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a, housing 7101. Here,the housing 7101 is supported by a stand 7105. Images can be displayedon the display portion 7103, and in the display portion 7103, thelight-emitting devices described in Embodiments 1 and 2 are arranged ina matrix.

The television device can be operated with an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, a general televisionbroadcast can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) data communication can beperformed.

FIG. 9B1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured using the light-emitting devices describedin Embodiments 1 and 2 and arranged in a matrix in the display portion7203. The computer illustrated in FIG. 981 may have a structureillustrated in FIG. 9B2. A computer illustrated in FIG. 9B2 is providedwith a second display portion 7210 instead of the keyboard 7204 and thepointing device 7206. The second display portion 7210 is a touch panel,and input operation can be performed by touching display for input onthe second display portion 7210 with a finger or a dedicated pen. Thesecond display portion 7210 can also display images other than thedisplay for input. The display portion 7203 may also be a touch panel.Connecting the two screens with a hinge can prevent troubles; forexample, the screens can be prevented from being cracked or broken whilethe computer is being stored or carried.

FIG. 9C illustrates an example of a portable terminal. A cellular phoneis provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the cellular phone hasthe display portion 7402 including the light-emitting devices describedin Embodiments 1 and 2 and arranged in a matrix.

When the display portion 7402 of the portable terminal illustrated inFIG. 9C is touched with a finger or the like, data can be input into theportable terminal. In this case, operations such as making a call andcreating an e-mail can be performed by touching the display portion 7402with a finger or the like.

The display portion 7402 has mainly three screen modes. The first modeis a display mode mainly for displaying images. The second mode is aninput mode mainly for inputting data such as text. The third mode is adisplay-and-input mode in which the two modes, the display mode and theinput mode, are combined.

For example, in the case of making a call or creating an e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on the screen can be input. In this case, itis preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a sensing device including a sensor for sensing inclination, suchas a gyroscope sensor or an acceleration sensor, is provided inside theportable terminal, display on the screen of the display portion 7402 canbe automatically changed by determining the orientation of the portableterminal (whether the portable terminal is placed horizontally orvertically).

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on the kind of imagesdisplayed on the display portion 7402. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed for a certain period while a signal sensed by anoptical sensor in the display portion 7402 is sensed, the screen modemay be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenwhen the display portion 7402 is touched with the palm or the finger,whereby personal authentication can be performed. Furthermore, byproviding a backlight or a sensing light source that emits near-infraredlight in the display portion, an image of a finger vein, a palm vein, orthe like can be taken.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 4 asappropriate.

As described above, the application range of the light-emittingapparatus having the light-emitting device described in Embodiments 1and 2 is extremely wide so that this light-emitting apparatus can beused in electronic devices in a variety of fields. By using thelight-emitting device described in Embodiments 1 and 2, an electronicdevice with high reliability can be obtained.

FIG. 10A is a schematic view illustrating an example of a cleaningrobot.

A cleaning robot 510 includes a display 5101 on its top surface, aplurality of cameras 5102 on its side surface, a brush 5103, andoperation buttons 5104. Although not illustrated, the bottom surface ofthe cleaning robot 5100 is provided with a tire, an inlet, and the like.Furthermore, the cleaning robot 5100 includes various sensors such as aninfrared sensor, an ultrasonic sensor, an acceleration sensor, apiezoelectric sensor, an optical sensor, and a gyroscope sensor. Thecleaning robot 5100 has a wireless communication means.

The cleaning robot 5100 is self-propelled, detects dust 5120, and sucksup the dust through the inlet provided on the bottom surface.

The cleaning robot 5100 can determine whether there is an obstacle suchas a wall, furniture, or a step by analyzing images taken by the cameras5102. When the cleaning robot 5100 detects an object that is likely tobe caught in the brush 5103 (e.g., a wire) by image analysis, therotation of the brush 5103 can be stopped.

The display 5101 can display the remaining capacity of a battery, theamount of collected dust, and the like. The display 5101 may display apath on which the cleaning robot 5100 has run. The display 5101 may be atouch panel, and the operation buttons 5104 may be provided on thedisplay 5101.

The cleaning robot 5100 can communicate with a portable electronicdevice 5140 such as a smartphone. The portable electronic device 5140can display images taken by the cameras 5102. Accordingly, an owner ofthe cleaning robot 5100 can monitor his/her room even when the owner isnot at home. The owner can also check the display on the display 5101 bythe portable electronic device 5140 such as a smartphone.

The light-emitting apparatus of one embodiment of the present inventioncan be used for the display 5101.

A robot 2100 illustrated in FIG. 10B includes an arithmetic device 2110,an illuminance sensor 2101, a microphone 2102, an upper camera 2103, aspeaker 2104, a display 2105, a lower camera 2106, an obstacle sensor2107, and a moving mechanism 2108.

The microphone 2102 has a function of detecting a speaking voice of auser, an environmental sound, and the like. The speaker 2104 has afunction of outputting sound. The robot 2100 can communicate with a userusing the microphone 2102 and the speaker 2104.

The display 2105 has a function of displaying various kinds ofinformation. The robot 2100 can display information desired by a user onthe display 2105. The display 2105 may be provided with a touch panel.Moreover, the display 2105 may be a detachable information terminal, inwhich case charging and data communication can be performed when thedisplay 2105 is set at the home position of the robot 2100.

The upper camera 2103 and the lower camera 2106 each have a function ofcapturing an image of the surroundings of the robot 2100. The obstaclesensor 2107 can detect an obstacle in the direction where the robot 2100advances with the moving mechanism 2108. The robot 2100 can move safelyby recognizing the surroundings with the upper camera 2103, the lowercamera 2106, and the obstacle sensor 2107. The light-emitting apparatusof one embodiment of the present invention can be used for the display2105.

FIG. 10C illustrates an example of a goggle-type display. Thegoggle-type display includes, for example, a housing 5000, a displayportion 5001, a speaker 5003, an LED lamp 5004, a connection terminal5006, a sensor 5007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 5008, a displayportion 5002, a support 5012, and an earphone 5013.

The light-emitting apparatus of one embodiment of the present inventioncan be used for the display portion 5001 and the display portion 5002.

FIG. 11 illustrates an example in which the light-emitting devicedescribed in Embodiments 1 and 2 is used for a table lamp which is alighting device. The table lamp illustrated in FIG. 11 includes ahousing 2001 and a light source 2002, and the lighting device describedin Embodiment 3 may be used for the light source 2002.

FIG. 12 illustrates an example in which the light-emitting devicedescribed in Embodiments 1 and 2 is used for an indoor lighting device3001. Since the light-emitting device described in Embodiments 1 and 2has high reliability, the lighting device can have high reliability.Furthermore, since the light-emitting device described in Embodiments 1and 2 can have a large area, the light-emitting device can be used for alarge-area lighting device. Furthermore, since the light-emitting devicedescribed in Embodiments 1 and 2 is thin, the light-emitting device canbe used for a lighting device having a reduced thickness.

The light-emitting device described in Embodiments 1 and 2 can also beused for an automobile windshield or an automobile dashboard. FIG. 13illustrates one mode in which the light-emitting device described inEmbodiments 1 and 2 is used for an automobile windshield and anautomobile dashboard. Display regions 5200 to 5203 each include thelight-emitting device described in Embodiments 1 and 2.

The display regions 5200 and 5201 are display devices which are providedin the automobile windshield and in which the light-emitting devicesdescribed in Embodiments 1 and 2 are incorporated. The light-emittingdevices described in Embodiments 1 and 2 can be formed into what iscalled a see-through display device, through which the opposite side canbe seen, by including an anode and a cathode formed of electrodes havinga light-transmitting property. Such see-through display devices can beprovided even in the automobile windshield without hindering the view.In the case where a driving transistor or the like is provided, atransistor having a light-transmitting property, such as an organictransistor including an organic semiconductor material or a transistorincluding an oxide semiconductor, is preferably used.

A display device incorporating the light-emitting device described inEmbodiments 1 and 2 is provided in the display region 5202 in a pillarportion. The display region 5202 can compensate for the view hindered bythe pillar by displaying an image taken by an imaging unit provided inthe car body. Similarly, the display region 5203 provided in thedashboard portion can compensate for the view hindered by the car bodyby displaying an image taken by an imaging unit provided on the outsideof the automobile. Thus, blind areas can be eliminated to enhance thesafety. Displaying images that compensate for the areas which a drivercannot see enables the driver to ensure safety easily and comfortably.

The display region 5203 can provide a variety of kinds of information.The content or layout of the display can be changed freely by a user asappropriate. Note that such information can also be displayed on thedisplay regions 5200 to 5202. The display regions 5200 to 5203 can alsobe used as lighting devices.

FIGS. 14A and 14B illustrate a foldable portable information terminal5150. The foldable portable information terminal 5150 includes a housing5151, a display region 5152, and a bend portion 5153. FIG. 14Aillustrates the portable information terminal 5150 that is opened. FIG.14B illustrates the portable information terminal 5150 that is folded.Despite its large display region 5152, the portable information terminal5150 is compact in sire and has excellent portability when folded.

The display region 5152 can be folded in half with the bend portion5153. The bend portion 5153 includes a flexible member and a pluralityof supporting members. When the display region is folded, the flexiblemember expands and the bend portion 5153 has a radius of curvature ofgreater than or equal to 2 mm, preferably greater than or equal to 3 mm.

Note that the display region 5152 may be a touch panel (an input/outputdevice) including a touch sensor (an input device). The light-emittingapparatus of one embodiment of the present invention can be used for thedisplay region 5152.

FIGS. 15A to 15C illustrate a foldable portable information terminal9310. FIG. 15A illustrates the portable information terminal 9310 thatis opened. FIG. 15B illustrates the portable information terminal 9310that is being opened or being folded. FIG. 15C illustrates the portableinformation terminal 9310 that is folded. The portable informationterminal 9310 is highly portable when folded. The portable informationterminal 9310 is highly browsable when opened because of a seamlesslarge display region.

A display panel 9311 is supported by three housings 9315 joined togetherby hinges 9313. Note that the display panel 9311 may be a touch panel(an input/output device) including a touch sensor (an input device). Byfolding the display panel 9311 at, the hinges 9313 between two housings9315, the portable information terminal 9310 can be reversibly changedin shape from the opened state to the folded state. The light-emittingapparatus of one embodiment of the present invention can be used for thedisplay panel 9311.

Example 1

Described in this example are fabrication methods and impedancespectroscopy measurement results of a light-emitting device 1 of oneembodiment of the present invention and a comparative light-emittingdevice 1. Structural formulae of organic compounds used for thelight-emitting device 1 and the comparative light-emitting device 1 areshown below.

(Fabrication Method of Light-Emitting Device 1)

First, indium tin oxide containing silicon oxide (ITSO) was depositedover a glass substrate by a sputtering method to form the anode 101. Thethickness of the anode 101 was 70 nm and the electrode area was 2 mm×2mm.

Next, in pretreatment for forming the light-emitting device over asubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure was reduced to approximately 10⁻⁴ Pa,vacuum baking was performed at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate wascooled down for approximately 30 minutes.

Next, the substrate provided with the anode 101 was fixed to a substrateholder provided in the vacuum evaporation apparatus such that the sideon which the anode 101 was formed faced downward. Then,N,N-bis(4-biphenyl)-6-phenylbenzo[b]naphtho[1,2-d]furan-8-amine(abbreviation: BBABnf) represented by Structural Formula (i) andALD-MP001Q (produced by Analysis Atelier Corporation, material serialNo. IS20180314) were deposited on the anode 101 to a thickness of 10 nmby co-evaporation using a resistance-heating method such that the weightratio of BBABnf to ALD-MP001Q was 1:0.1, whereby the hole-injectionlayer 111 was formed. Note that ALD-MP001Q is an organic compound havingan acceptor property.

Subsequently, over the hole-injection layer 111, BBABnf was deposited byevaporation to a thickness of 20 nm to form the first hole-transportlayer 112-1, and then3,3′-(naphthalene-1,4-diyl)bis(9-phenyl-9H-carbazole) (abbreviation:PCzN2) represented by Structural Formula (ii) was deposited byevaporation to a thickness of 10 nm to form the second hole-transportlayer 112-2, whereby the hole-transport layer 112 was formed. Note thatthe second hole-transport layer 112-2 also functions as anelectron-blocking layer.

Then, 9-(1-naphthyl)-10-[4-(2-naphthyl)phenyl]anthracene (abbreviation:αN-βNPAnth) represented by Structural Formula (iii) and3,10-bis[N-(9-phenyl-9H-carbazol-2-yl)-N-phenylamino]naphthol[2,3-b;6,7-b′]bisbenzofuran(abbreviation: 3,10PCA2Nbl(IV)-02) represented by Structural Formula(iv) were deposited by co-evaporation to a thickness of 25 nm such thatthe weight ratio of αN-βNPAnth to 3,10PCA2Nb(IV)-02 was 1:0.015, wherebythe light-emitting layer 113 was formed.

Then, over the light-emitting layer 113,2-{4-[9,10-di(naphthalen-2-yl)-2-anthryl]phenyl}-1-phenyl-1H-benzimidazole(abbreviation: ZADN) represented by Structural Formula (v) and8-hydroxyquinolinato-lithium(abbreviation. Liq) represented byStructural Formula (vi)(produced by Chemipro Kasei Kaisha, Ltd., serialNo. 181201) were deposited by co-evaporation to a thickness of 12.5 nmsuch that the weight ratio of ZADN to Liq was 0.7:1, and successively,ZADN and Liq were deposited by co-evaporation to a thickness of 12.5 nmsuch that the weight ratio of ZADN to Liq was 1:0.7, whereby theelectron-transport layer 114 was formed.

After the formation of the electron-transport layer 114, Liq wasdeposited by evaporation to a thickness of 1 nm to form theelectron-injection layer 115. Then, aluminum was deposited byevaporation to a thickness of 200 nm to form the cathode 102. Thus, thelight-emitting device 1 of this example was fabricated.

(Fabrication Method of Comparative Light-Emitting Device 1)

The comparative light-emitting device 1 was fabricated in the samemanner as the light-emitting device 1 except that αN-βNPAnth in thelight-emitting layer 113 is replaced with7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by Structural Formula (vii) and theelectron-transport layer 114 was formed by evaporation of2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (viii)to a thickness of 15 nm and then evaporation of2,9-di(2-naphthyl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBPhen) represented by Structural Formula (ix) to a thickness of 10 nm.

The structures of the light-emitting device 1 and the comparativelight-emitting device t are listed in the following table.

TABLE 3 Light-emitting device 1 Hole- Electron- injection Hole-transportlayer Electron-transport layer injection layer 1 2 Light-emitting layer1 2 layer 10 nm 20 nm 10 nm 25 nm 12.5 nm 12.5 nm 1 nm BBABnf:ALD-BBABnf PCzN2 αN-βNPAnth:3, ZADN:Liq Liq MP001Q 10PCANbf(IV)-02 (0.7:1)(1:0.7) (1:0.1) (1:0.015) Comparative light-emitting device 1 Hole-Electron- injection Hole-transport layer Electron-transport layerinjection layer 1 2 Light-emitting layer 1 2 layer 10 nm 20 nm 10 nm 25nm 15 nm 10 nm 1 nm BBABnf:ALD- BBABnf PCzN2 cgDBCzPA:3, 2mDBTBPDBq-IINBPhen Liq MP001Q 10PCANbf(IV)-02 (1:0.1) (1:0.015)

The HOMO levels, the LUMO levels, and the electron mobilities of theorganic compounds used in this example are listed in the followingtable. The electron mobilities were measured when the square root of theelectric field strength [V/cm] was 600.

TABLE 4 HOMO level LUMO level Electron mobility (eV) (eV) (cm²/Vs)BBABnf −5.56 — — PCzN2 −5.71 — — αN-βNPAnth −5.85 −2.74 — ZADN — −2.87 —ZADN:Liq (1:1) — — 3.5 × 10⁻⁶ cgDBCzPA −5.69 −2.74 7.7 × 10⁻⁵2mDBTBPDBq-II — −2.94 2.2 × 10⁻⁶

The light-emitting devices were sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (a sealant was applied to surround the devices and UV treatmentand heat treatment at 80° C. for 1 hour were performed at the time ofsealing). Then, the initial characteristics and reliability of thelight-emitting devices were measured. Note that the measurement wasperformed at room temperature. Table 5 shows the main characteristics ofthe light-emitting device 1 and the comparative light-emitting device 1at a luminance of about 1000 cd/m².

TABLE 5 Current Current External Voltage Current density ChromaticityChromaticity efficiency quantum (V) (mA) (mA/cm²) x y (cd/A) efficiency(%) Light-emitting device 1 3.1 0.32 8.1 0.14 0.13 12.0 11.5 Comparativelight- 4.4 0.29 7.2 0.14 0.13 10.6 10.2 emitting device 1

Table 5 shows that the light-emitting device 1 of one embodiment of thepresent invention and the comparative light-emitting device 1 areblue-light-emitting devices with favorable initial characteristics.

FIG. 20 is a graph showing a change in luminance over driving time at acurrent density of 50 mA/cm². As shown in FIG. 20, the luminance of thelight-emitting device 1 of one embodiment of the present inventionincreases after the start of the driving, becomes higher than theinitial luminance, and then gradually decreases. That is, the decaycurve has the maximum value. This results in a significant improvementin the driving lifetime particularly in the state with a small decay ofapproximately 2% to 5% with respect to the initial luminance.

Next, the light-emitting device 1 and the comparative light-emittingdevice 1 were subjected to impedance spectroscopy measurement (ISmeasurement). A micro sinusoidal voltage signal (V=V₀[exp(iωt)]) wasapplied to the light-emitting devices, and the impedance (Z=V/I) wasobtained from a phase difference between the current amplitude of aresponse current signal (I=I₀exp[(i(ωt+ϕ)]) and the input signal. Byapplying the voltage to the light-emitting device while the frequency ofthe voltage is changed from a high level to a low level, componentshaving various relaxation times that contribute to the impedance can beseparated and measured.

The obtained impedance is plotted on the complex plane (a Nyquist plot)with the frequency of the applied voltage signal used as a parameter.From the impedance (Z), admittance (Y), modulus (M), dielectric constant(ε), and the like, which are basic transfer functions, can be obtained.

In this example, the light-emitting devices were analyzed using animpedance (Z) plot, the real axis of which gives a resistance, and amodulus (M) plot, which gives the reciprocal number of a capacitance.

The measurement was performed with a cable for measurement of anextremely low current combined with a high-performance electrochemicalmeasurement system, SP-300 (advanced model) produced by BioLogic.

FIGS. 21A and 21B show the Z plots of the light-emitting device 1 andthe comparative light-emitting device 1, respectively. In FIGS. 21A and21B, the measurement was performed with a frequency ranging from 1 MHzto 3 MHz, an AC voltage of 100 mV, and an applied voltage of 2.5 V. Toobtain the devices after driving, the light-emitting device 1 was drivenfor 670 hours at 50 mA/cm and the comparative light-emitting device 1was driven for 380 hours at 50 mA/cm.

FIG. 21A shows that the resistance of the light-emitting device 1 of oneembodiment of the present invention is lower after driving than beforedriving. In contrast, FIG. 21B shows that the resistance of thecomparative light-emitting device 1, which is a conventional device, isslightly higher after driving than before driving.

FIG. 22 shows a change in voltage over the driving time at a constantcurrent density of 50 mA/cm². In FIG. 22, the driving voltage of thelight-emitting device 1 of one embodiment of the present inventiondecreases in driving at a constant current density, which shows theresult consistent with the IS measurement result that indicates adecrease in resistance due to driving.

Note that the driving voltage of the comparative light-emitting device1, which is a conventional device, increases with driving, which alsoshows the result consistent with the IS measurement result. Almost allthe conventional light-emitting devices tend to have such a drivingvoltage that increases with driving at a constant current density. Thedecrease in driving voltage with driving is a unique feature of thelight-emitting device of one embodiment of the present invention.

Next, the M plot of the light-emitting device 1 is shown in FIG. 23. Thecurve shape before driving is different from that after driving, whichsuggests that the modulus changes before and after driving.

Then, the graph was fitted using equivalent circuit analysis software,ZView (Scribner Associates Inc., US), offering an equivalent circuit ofthe light-emitting device, which consists of four RC parallel circuitsand one series resistance as shown in FIG. 18. Note that the numbers inthe M plot represent the corresponding resistances placed in theequivalent circuit separated by fitting.

FIG. 19 is a graph obtained by plotting the separated resistance valuesbefore and after driving. This indicates that only the resistance R2after driving is lower by one digit or more than that before driving.

To examine which layer in the light-emitting device 1 corresponds toeach resistance, light-emitting devices having layers with differentthicknesses (light-emitting devices 2 to 8) were fabricated andsubjected to the IS measurement. Note that these light-emitting deviceshave the same structure as the light-emitting device 1 except for thecomposition of the electron-transport layer 114 and the thicknesses withboldface numbers in the table below. As for the electron-transport layer114, the light-emitting device 1 has a two-layer structure of the12.5-nm-thick first electron-transport layer 114-1 (ZADN:Liq=0.7:1) andthe 12.5-nm-thick second electron-transport layer 114-2(ZADN:Liq=1:0.7), whereas the light-emitting devices 2 to 5 include asingle electron-transport layer (ZADN:Liq 1:1) with a thickness of 25nm, 35 nm, 45 nm, and 55 nm, respectively, and the light-emittingdevices 6 to 8 include a single electron-transport layer (ZADN:Liq=1:1)with a thickness of 25 nm.

TABLE 6 Light First hole- emitting Electron- transport layer layertransport layer (nm) (nm) (nm) Light-emitting device 20 25 25 2Light-emitting device 20 25 35 3 Light-emitting device 20 25 45 4Light-emitting device 20 25 55 5 Light-emitting device 20 40 25 6Light-emitting device 20 55 25 7 Light-emitting device 50 25 25 8

The IS measurement was performed on the light-emitting devices 2 to 8and the M plots were made on the basis of the obtained results (FIGS.24A and 24B, FIGS. 25A and 25B, and FIGS. 26A and 26B). FIG. 24A showsthe M plots of the light-emitting devices 3 to 5 with differentthicknesses of the electron-transport layer 114, and the referencelight-emitting device 2; FIG. 25A, the M plots of the light-emittingdevices 6 and 7 with different thicknesses of the light-emitting layer113, and the reference light-emitting device 2; and FIG. 26A, the Mplots of the light-emitting device 8 with a different thickness of thefirst hole-transport layer 112-1, and the reference light-emittingdevice 2.

FIGS. 24A, 25A, and 26A show that the shape of the M plot changes withthe thickness of each layer. Note that in FIGS. 24B, 253B, and 26B, theresistances of the light-emitting devices are plotted for each ofresistances R1 to R5, which correspond to those in the equivalentcircuit obtained by fitting using ZView.

FIGS. 24A and 24B, FIGS. 25A and 25B, and FIGS. 26A and 26D show thatthe resistance R2 changes in the light-emitting device with a differentthickness of the electron-transport layer 114, the resistance R1 changesin the light-emitting device with a different thickness of thelight-emitting layer 113, and the resistance R3 changes in thelight-emitting device with a different thickness of the firsthole-transport layer 112-1. Hence, in the equivalent circuit of thelight-emitting device of one embodiment of the present invention shownin FIG. 18, R1, R2, and R3 were found to correspond to thelight-emitting layer, the electron-transport layer, and the firsthole-transport layer, respectively, which suggests that R4 is the secondhole-transport layer.

From the above, the resistance R2, which changes before and afterdriving, was found to be derived from the electron-transport layer 114.

As described above, the light-emitting device 1 of one embodiment of thepresent invention has a long lifetime and the resistance of the wholelight-emitting device 1 after driving is lower than that before driving.This decrease in resistance was found to be derived from the decrease inthe resistance of the electron-transport layer 114.

From the above results, when the light-emitting device 1 is driven, theresistance of the electron-transport layer 114 decreases to allowcarriers (electrons) to easily flow, so that the end of therecombination region, which has extended to part of theelectron-transport layer 114, moves toward the light-emitting region. Asa result, the energy of recombination, which has been deactivatedwithout contributing to light emission in the electron-transport layer,can be converted into light emission, which probably causes theluminance increase as shown in FIG. 20.

The light-emitting device of one embodiment of the present invention canhave a long lifetime as described above.

Example 2

Described in this example are examples of a material capable of beingused for the electron-transport layer 114, whose resistance decreaseswhen current flows (when a light-emitting device is driven).

As the material whose resistance decreases with current flowingtherethrough, an organometallic complex of an alkali metal or analkaline earth metal, or a mixture material of an electron-transportorganic compound and an organometallic complex of an alkali metal or analkaline earth metal can be preferably used. In this example, thecalculation results using2-phenyl-3-{4-[10-(3-pyridyl)-9-anthryl]phenyl}quinoxaline(abbreviation: PyAlPQ) as the electron-transport organic compound, andLiq as the organometallic complex of an alkali metal or an alkalineearth metal, will be shown.

First, the ease of formation of a multimer of Liq in a film where PyAlPQand Liq were mixed at a weight ratio of 1:1 was calculated by aclassical molecular dynamics simulation. Specifically, PyAlPQ and Liqmolecules were put in a cell so as to have a molar ratio of 18:82 basedon a weight ratio of 1:1; the cell was compressed at high temperatureand pressure; and the pressure and the temperature were reduced toatmospheric pressure and room temperature, respectively, to relieve thecell, so that an amorphous model was obtained. When the condensation ofmolecules in the cell was examined, association of Liq, such as a dimeror a trimer, and a cluster of Liq, such as a hexamer were extracted.This indicates that the multimer of Liq is easily formed even at roomtemperature and in atmospheric pressure.

Then, the stability of a Liq molecule alone and the stability of amultimer thereof were compared by structure optimization using firstprinciples calculation. The structure optimization calculation wasperformed under vacuum conditions using Gaussian 09, Revision E.01 andRB3LYP/6-311g(d,p) as a basis function. The examination was performed asfollows: the energy of a Liq molecule alone was compared with the energyof multimers of Liq; the amount of energy stabilized by multimerizationwas calculated and divided by the number of Liq molecules; and thestabilized energy per Liq molecule was calculated. The calculation andexamination were performed on a dimer, a trimer, a tetramer, a hexamer,and an octamer of Liq. The results are shown in FIG. 27.

From the graph of FIG. 27, the hexamer of Liq is the most stable and thestabilized energy per Liq molecule is approximately 1.54 eV. Thisindicates that the hexamer of Liq has a stable structure and is likelyto be formed preferentially. According to the calculation, there is nolarge difference in stabilization energy of the tetramer to the octamerof Liq, which means that the tetramer or more are easily generated asthe multimers.

Thus, multimers of Liq which is used in the electron-transport layer,are probably formed gradually as the light-emitting device is driven(current flows). That is, in the material whose resistivity decreaseswith current flowing therethrough, multimers of the organometalliccomplex of an alkali metal or an alkaline earth metal are graduallyformed to increase the electron mobility and reduce the resistance.

In the light-emitting device of one embodiment of the present invention,the carrier balance is adjusted so that the recombination region reachesthe electron-transport layer at the initial driving stage; as thelight-emitting device continues to be driven, a cluster of theorganometallic complex of an alkali metal or an alkaline earth metal isformed in the electron-transport layer, whereby the electron mobilityincreases and the carrier balance changes. As a result, therecombination region falls within the light-emitting layer to increasethe luminance, allowing the light-emitting device to have a longlifetime and a small initial decay.

Reference Example 1

In this reference example, methods for calculating the HOMO levels, theLUMO levels, and the electron mobilities of the organic compounds usedin the examples are described.

The HOMO level and the LUMO level can be calculated through cyclicvoltammetry (CV) measurement.

An electrochemical analyzer (ALS model 600A or 600C, manufactured by BASInc.) was used as the measurement apparatus. A solution for the CVmeasurement was prepared in the following manner: tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, produced by Tokyo Chemical Industry Co., Ltd.,catalog No. T0836) as a supporting electrolyte was dissolved indehydrated dimethylformamide (DMF, produced by Sigma-Aldrich Co. LLC.,99.8%, catalog No. 22705-6) as a solvent at a concentration of 100mmol/L, and the object to be measured was dissolved therein at aconcentration of 2 mmol/L. A platinum electrode (PTE platinum electrode,manufactured by BAS Inc.) was used as a working electrode, anotherplatinum electrode (Pt counter electrode for VC-3 (5 cm), manufacturedby BAS Inc.) was used as an auxiliary electrode, and an Ag/Ag⁺ electrode(RE7 reference electrode for nonaqueous solvent, manufactured by BASInc.) was used as a reference electrode. Note that the measurement wasconducted at room temperature (20° C. to 25° C.). In addition, the scanspeed in the CV measurement was fixed to 0.1 V/sec, and an oxidationpotential Ea [V] and a reduction potential Ec [V] with respect to thereference electrode were measured. The potential Ea is an intermediatepotential of an oxidation-reduction wave, and the potential Ec is anintermediate potential of a reduction-oxidation wave. Here, since thepotential energy of the reference electrode used in this example withrespect to the vacuum level is known to be −4.94 [eV], the HOMO leveland the LUMO level can be calculated by the following formulae: HOMOlevel [eV]=−4.94−Ea and LUMO level [eV]=−4.94−Ec.

The electron mobility can be measured by an impedance spectroscopy (IS)method.

As a method for measuring the carrier mobility of an EL material, atime-of-flight (TOF) method, a method using I-V characteristics of aspace-charge-limited current (SCLC), or the like has been known for along time. The TOF method needs a sample with a much larger thicknessthan that of an actual organic EL device. The SCLC method has adisadvantage in that electric field strength dependence of carriermobility cannot be obtained, for example. Since an organic film requiredfor the measurement employing the IS method is thin (approximatelyseveral hundreds of nanometers), the organic film can be formed of arelatively small amount of EL materials, whereby the mobility can bemeasured with a thickness close to the thickness of a film in an actualEL device. In this method, the electric field strength dependence of thecarrier mobility can also be measured.

In the IS method, a micro sinusoidal voltage signal (V=V₀[exp(jωt)]) isapplied to an EL device, and the impedance of the EL device (Z=V/I) isobtained from a phase difference between the current amplitude of aresponse current signal (I=I₀exp[j(ωt+ϕ)]) and the input signal. Byapplying the voltage to the EL device while the frequency of the voltageis changed from a high level to a low level, components having variousrelaxation times that contribute to the impedance can be separated andmeasured.

Here, admittance Y (=1/Z), which is the reciprocal number of theimpedance, can be represented by conductance G and susceptance B asshown in the following formula (1).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{Y = {\frac{1}{Z} = {G + {jB}}}} & (1)\end{matrix}$

In addition, by a single injection model, calculation of the followingformulae (2) and (3) can be performed. Here, g in the formula (4) isdifferential conductance. In the formula. C represents capacitance, θrepresents a transit angle (ωt), ω represents angular frequency, and trepresents transit time. For the analysis, the current equation, thePoisson's equation, and the current continuity equation are used, and adiffusion current and a trap state are ignored.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{G = {\frac{g\;\theta^{3}}{6}\frac{\theta - {\sin\;\theta}}{\left( {\theta - {\sin\;\theta}} \right)^{2} + \left( {\begin{matrix}\theta^{2} \\2\end{matrix} + {\cos\;\theta} - 1} \right)^{2}}}} & (2) \\{B = {{\omega\; C} = {\frac{g\;\theta^{3}}{6}\frac{\frac{\theta^{2}}{2} + {\cos\;\theta_{1}}}{\left( {\theta - {\sin\;\theta}} \right)^{2} + \left( {\frac{\theta^{2}}{2} + {\cos\;\theta} - 1} \right)^{2}}}}} & (3) \\{g = {\frac{9}{4}ɛ\;\mu\frac{V_{0}}{d^{3}}}} & (4)\end{matrix}$

A method for calculating mobility from the frequency characteristics ofcapacitance is a −ΔB method. A method for calculating mobility from thefrequency characteristics of conductance is a ωΔG method.

In practice, first, a measurement device is fabricated using a materialwhose electron mobility is intended to be calculated. The measurementdevice is designed such that only electrons flow therein as carriers.FIG. 28 is a schematic view of a measurement device used in thisexample. In this specification, a method for calculating electronmobility from the frequency characteristics of capacitance (the −ΔBmethod) is described.

As illustrated in FIG. 28, the measurement device fabricated in thisexample includes a first layer 210, a second layer 211, and a thirdlayer 212 between an anode 201 and a cathode 202. The material whoseelectron mobility is intended to be calculated is used as a material forthe second layer 211. For explanation, an example in which the electronmobility of a film formed by co-evaporation of ZADN and Liq in a weightratio of 0.5:0.5 is measured is given. A specific structure example islisted in the following table.

TABLE 7 First Second Third Anode layer layer layer Cathode 100 nm 50 nm100 nm 1 nm 200 nm 1 nm 100 nm APC NITO Al Liq ZADN:Liq Liq Al (0.5:0.5)

The impedance was measured under the conditions where the frequency was1 Hz to 3 MHz, the AC voltage was 70 mV, and the DC voltage was appliedin the range of 5.0 V to 9.0 V. Here, capacitance is calculated fromadmittance, which is the reciprocal number of the obtained impedance(the above formula (1)). FIG. 29 shows the frequency characteristics ofthe calculated capacitance C when the application voltage was 7.0 V.

The frequency characteristics of the capacitance C are obtained from aphase difference in current, which is generated because a space chargegenerated by carriers injected by the micro voltage signal cannotcompletely follow the micro AC voltage. The transit time of the injectedcarriers in the film is defined by time T until the carriers reach acounter electrode, and is represented by the following formula (5).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\{T = {\frac{4}{3}\frac{L^{2}}{\mu\; V_{0}}}} & (5)\end{matrix}$

A negative susceptance change (−ΔB) corresponds to a value (−ωΔC)obtained by multiplying a capacitance change −ΔC by angular frequency ω.According to the formula (3), there is a relation between peak frequencyon the lowest frequency side f_(max) (=ω_(max)/2π) and the transit timeT as shown in the following formula (6).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack & \; \\{T = \frac{4.5}{2\;\pi\; f_{\max}^{\prime}}} & (6)\end{matrix}$

FIG. 30 shows the frequency characteristics of −ΔB calculated from theabove measurement (i.e., −ΔB at a DC voltage of 7.0 V). The peakfrequency on the lowest frequency side f_(max) is indicated by an arrowin FIG. 30.

The transit time T is obtained from f_(max) obtained from the abovemeasurement and analysis (see the above formula (6)); thus, in thisexample, the electron mobility at a DC voltage of 7.0 V can be obtainedfrom the above formula (5). Through the same measurement with the DCvoltage in the range of 5.0 V to 9.0 V, the electron mobility at eachvoltage (electric field strength) can be calculated, so that theelectric field strength dependence of the mobility can also be measured.

FIG. 31 shows the final electric field strength dependence of theelectron mobility of the organic compounds obtained by the abovecalculation method, and Table 9 shows the values of the electronmobility in the case where the square root of the electric fieldstrength [V/cm] read from the figure was 600 [V/cm]^(1/2).

TABLE 8 Electron mobility (cm²/Vs) cgDBCzPA 7.7 × 10⁻⁵ 2mDBTBPDBq-II 2.2× 10⁻⁵ ZADN:Liq (1:1) 3.5 × 10⁻⁶

The electron mobility can be calculated as described above. For thedetails about the measurement method, refer to the following reference:T. Okachi et al., Japanese Journal of Applied Physics, vol. 47, No. 12,pp. 8965-8972, 2008.

Reference Example 2

A synthesis methods of2-phenyl-3-{4-[10-(3-pyridyl)-9-anthryl]phenyl}quinoxaline(abbreviation: PyAlPQ) used in Example 2 will be described. Thestructure of PyAlPQ is shown below.

Into a 50 mL three-neck flask were added 0.74 g (2.2 mmol) of3-(10-bromo-9-anthryl)pyridine, 0.26 g (0.85 mmol) oftri(ortho-tolyl)phosphine, 0.73 g (2.3 mmol) of4-(3-phenylquinoxalin-2-yl)phenylboronic acid, 1.3 g (9.0 mmol) of anaqueous solution of potassium carbonate, 40 mL of ethylene glycoldimethyl ether (DME), and 4.4 mL of water. The mixture was degassed bybeing stirred under reduced pressure, and the air in the flask wasreplaced with nitrogen.

To the mixture in the flask was added 65 mg (0.29 mmol) of palladium(II) acetate, and the mixture was stirred under a nitrogen stream at 80°C. for 11 hours. After the stirring, water was added to the mixture inthe flask, followed by extraction with toluene. The obtained solution ofthe extract was washed with saturated brine, and drying with magnesiumsulfate was performed. The mixture was gravity filtered, and thefiltrate was concentrated to give an oily substance. The obtained oilysubstance was purified twice by silica gel column chromatography, firstusing chloroform and then using toluene:ethyl acetate=5:1 andrecrystallized with toluene/hexane, giving 0.43 g of a target yellowsolid in a yield of 36%. The synthesis scheme is shown below.

By a train sublimation method, 0.44 g of the obtained yellow solid waspurified by sublimation. In the purification by sublimation, the solidwas heated at 260° C. for 18 hours under a pressure of 10 Pa with anargon gas flow rate of 5.0 mL/min. After the purification bysublimation, 0.35 g of a target yellow solid was obtained at acollection rate of 79%.

Analysis results by nuclear magnetic resonance (¹H-NMR) spectroscopy ofthe yellow solid obtained by the above reaction are shown below. Theresults reveal that PyAlPQ represented by the above structural formulawas obtained.

¹H NMR (CDCl3, 300 MHz): δ=7.37-7.50 (m, 9H), 7.56-7.78 (m, 9H),7.82-7.86 (m, 3H), 8.24-8.30 (m, 2H), 8.75 (dd, J=1.8 Hz, 0.9 Hz, 1H),8.84 (dd, J=4.8 Hz, 1.8 Hz, 1H).

This application is based on Japanese Patent Application Serial No.2019-110831 filed with Japan Patent Office on Jun. 14, 2019, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting device comprising: an anode; acathode; and an EL layer between the anode and the cathode, wherein theEL layer includes a hole-injection layer, a light-emitting layer, and anelectron-transport layer, wherein the hole-injection layer is positionedbetween the anode and the light-emitting layer, wherein theelectron-transport layer is positioned between the light-emitting layerand the cathode, wherein the hole-injection layer contains a firstsubstance and a second substance, wherein the first substance is anorganic compound which has a hole-transport property and a HOMO levelhigher than or equal to −5.7 eV and lower than or equal to −5.4 eV,wherein the second substance exhibits an electron-accepting propertywith respect to the first substance, and wherein the electron-transportlayer contains a material whose resistance decreases with currentflowing therethrough.
 2. The light-emitting device according to claim 1,wherein the material whose resistance decreases with current flowingtherethrough contains an organometallic complex of an alkali metal or analkaline earth metal.
 3. The light-emitting device according to claim 1,wherein the material whose resistance decreases with current flowingtherethrough contains an organic compound having an electron-transportproperty and an organometallic complex of an alkali metal or an alkalineearth metal.
 4. The light-emitting device according to claim 2, whereinthe organometallic complex of an alkali metal or an alkaline earth metalforms a cluster.
 5. The light-emitting device according to claim 2,wherein the organometallic complex of an alkali metal or an alkalineearth metal is a metal complex including a ligand containing nitrogenand oxygen and an alkali metal or an alkaline earth metal.
 6. Thelight-emitting device according to claim 2, wherein the organometalliccomplex of an alkali metal or an alkaline earth metal is a metal complexincluding a monovalent metal ion and a ligand having an8-hydroxyquinolinato structure.
 7. The light-emitting device accordingto claim 2, wherein the organometallic complex of an alkali metal or analkaline earth metal is a lithium complex including a ligand having an8-hydroxyquinolinato structure.
 8. The light-emitting device accordingto claim 2, wherein the electron-transport layer includes a first layerand a second layer, wherein the first layer is positioned between thelight-emitting layer and the second layer, wherein the second layer ispositioned between the first layer and the cathode, and wherein aconcentration of the organometallic complex of an alkali metal or analkaline earth metal in the first layer is different from that in thesecond layer.
 9. The light-emitting device according to claim 8, whereinthe concentration of the organometallic complex of an alkali metal or analkaline earth metal in the first layer is higher than that in thesecond layer.
 10. The light-emitting device according to claim 1,wherein the second substance is an organic compound.
 11. Thelight-emitting device according to claim 1, wherein the light-emittinglayer contains a host material and a light-emitting substance, andwherein the light-emitting substance emits blue fluorescence.
 12. Anelectronic device comprising: the light-emitting device according toclaim 1; and a sensor, an operation button, a speaker, or a microphone.13. A light-emitting apparatus comprising: the light-emitting deviceaccording to claim 1; and a transistor or a substrate.
 14. A lightingdevice comprising: the light-emitting device according to claim 1; and ahousing.